Tarpit

In 40K terminology, a Tarpit is a big blob of expendable infantry intended to be a giant pain in the ass and functionally a living wall designed to slow enemies down and keep foes tied up by either assaulting them or act as a wall of dakka. Ideally capable of being placed in cover, almost every faction in the the 41st Millennium is capable of fielding some unit or another that serves this purpose. Some are especially good at it, whilst others are simply mediocre at it.

Tarpit Units By Army

Below is a brief list of some classic Tarpit units favored by various factions in the 40K universe.

Imperial Guard

• Conscript Platoon - 50+ ( concripts are now 3 pts a piece LOL!) Models for dirt cheap. Do the math. Get all 50 for 200 points and stick a Lord Commissar so they will never, ever go away. Alternatively, get Commander Chenkov and watch as you drown your enemy in your never-ending waves of conscript hordes. (personal note: i once had a squad of 50 conscripts, and the DEMOLISHED a 20 man squad of khorne bezerkers. with overwatch. OVERWATCH! These guys rock! Just saying.)
• Regular Guardsmen - They're just a point higher than conscripts. If you want some guys who can fight a little better and stand as fuckheug blob, these are your guys. Terrifyingly shooty and you can actually arm them to be useful in assault. Combined squad of 5 Guardsman Squads can slaughter terminators in overwatch... Not quite, only one would go down statistically. However, I, unlike my friend here, got lucky and took down 5 assault terminators armed with thunder hammers and storm shields, as well as a terminator librarian, in overwatch. WITH LASGUNS. Miracles do happen people. If your dice rolls are lucky, you should usually take lots of guardsmen and tanks.
• Chimera APC - Cheap as shit and easy to get for some beefy armor that will tend to stick around all game long. Once they're done transporting, feel free to have your entire force group of Chimeras gang up and proceed to start roving in angry packs like a gang of 1920s Irish immigrants dealing with abuses by the police.
• Taking a Dark Angels allied detachment with Azrael will give a 50-man guard platoon 4++ saves and furious charge, ensuring that nothing short of top-tier firepower will get rid of them.

Chaos Daemons

• Nurglings - Gets into close-combat and keeps enemies tied up for half a game or longer.

Tau

• Kroot Carnivores - Inexpensive infiltrating models at 6 points base. Hell of an area denial unit that can frustrate anyone trying to deepstrike that Dreadnought near your battlesuit formation.
• Gue'vesa Auxillaries - Even less expensive and can't infiltrate but shooty and with decent wargear options.

Tyranids

• Ripper Swarm - Vulnerable to Instant Death, blast templates, and they die without synapse, but will tie foes up for round after round if they get close for a low price.
• Gaunts - Cheap and effective mass unit. When used with Tervigons they can stay around forever and ever and ever.

Orks

• Nearly every type of cheap yet numerous Ork Boy squad (Sluggas, shootas, et cetera). Sluggas works best at tying up enemy units overall though, a mob with 4 S4 attacks and near marine toughness will keep all but the 'ardest and killiest units from quickly getting rid of them. These guys are also your main troop choice as well, and will drown your enemies in close combat attacks as well as bodies.
• Gretchin - Dies like bitches to nearly everything, but numerous, can hold objectives, dirt-cheap, and can actually shoot better than most Ork boyz (so can anything with their eyes open (with less dakka)). Their prime use is pretty much to act as a walking meatshield in front of your sluggas so they won't go down on the initial charge on your opponents formidable gunline... Yo dawg, we heard you like tarpits, so we put a tarpit on yo' tarpit so you can tarpit while you tarpit... well, unless your opponent is packing a retarded amount of arty, which kinda renders this no more than a minor inconvenience for them.
• Take Mad Dok Grotsnik and for a bump in cost make the Tarpit unit cyborked, giving that large annoying tarpit unit 5+ invulnerable saves, this even applies to Gretchin. It costs a bit, but when your models are this cheap its worth it just to see that smug smirk die when that IG player's Leman Russ tank kills only 6 gretchin, or a Defiler fails to kill a single Boy in close combat before the Powerklaw tears it in half.

Necrons

• Scarab Swarm - Vulnerable to instant death/blast temps, but cheap for 30 wounds in a squad.
• Warriors - When DON'T you have a metric fuckton of warriors while playing as 'cronz? Very good if paired with the right formations and/or a Necron Lord with a resurrection Orb and is capable of Gauss-glancing any vehicle into dust in one round of shooting. A lord with res orb and a nearby ghost ark will make sure that these dudes don't go down any time soon.

Eldar

• Guardian squad - They're the cheapest and most numerous infantry squad you could field in an Eldar army and attaching a warlock with either embolden or conceal could allow them to do their jobs better at a low point cost, though honestly Eldar really shouldn't be tarpitting unless you're in a desperate situation. They're better off shooting than knifing someone to death though, unless you use Storm Guardians.

• Wraithblades aren't cheap but they have the T of a Riptide, MEQ armour saves and can take a 4++ for free. They also don't run away and hit back with high S power weapons. A big squad will hold up anything without lots of High S or Poisoned attacks for ever and deal back alot of damage.

Dark Eldar

• Wyches - They're a mildly effective close combat unit in their own right, but loading them up with Shardnets reduces enemy attacks, and they all get an invulnerable save. A great option for swamping Terminators and the like, and can actually beat a softened up terminator squad with a couple of lucky rolls due to the cost:attacks ratio between wyches and terminators (though this isn't nearly as effective due to 6th edition power weapon rules).
• Grotesques - While not really a tarpit, they're mad tough, especially if led by a Haemonculus, and can tie up MEQ and weaker infantry units with ease; they could probably tie them up for at least half the game or kill them outright.

Space Marines

• Scouts - They infiltrate, they can pump a good amount of dakka downrange, and about the cheapest unit you can field in an SM army.
• Rhino Transports - Once your Rhino already unloaded its troops, it functions as an annoying well-armored blockade/brick of cover that pretty much prevents your enemy from properly assaulting or targeting their intended target, park them in front of a squad about to be charge or taken out by a heavy weapons team - voila, instant cover. And to boot, it's cheap and still has a Storm Bolter to threaten a few weedy gits.

Space Wolves

• Fenrisian Wolves - While they're fast attack instead of troops, they're cheap, comes with a handy 6+ save and aren't that bad in melee.

Grey Knights

• Various inquisitorial henchmen - You have plenty of choices, but considering these guys are just fodder, go all in with Crusaders. But if you want to make things funnier, go all in with Jokaero and make planet of the apes references while playing, though note that this isn't the best competitive idea due to their price (although it means you'll field as much lascannons, heavy flamers and multi melta. If you're against a mechanized force with hordes of GEQ, they may be boned). But whatever happens, the Planet of the Apes reference will be TOTALLY worth it.
• Castellan Crowe - He would stick in combat like forever with his rerollable 2+/3++ save, while burning shit to death with his bare mind.

Chaos Space Marines

• Cultists - If you've played any army requiring mass infantry formations, you know what to do.
• Plague Marines - Papa Nurgle's good ol' T5/FnP plagues, these guys will never fucking die until someone whips out high-powered anti-armor weaponry. Plonk 7 of them and watch them weather out phase after phase of attacks. Seriously, seeing Plagues tank 30+ Orks with only a single casualty across three rounds is far from unheard of.
• Thousand Sons - Expensive, much as the Plague Marines are, but definitely a worthy entry. They aren't quite as tough to kill against small arms fire as Plagues, but they'll survive heavier firepower with ease and will basically laugh off power weapons, which Plague Marines won't. They also hose out a lot more dakka than a Plague squad. Watching an enemy nearly shit themselves after dropping a Demolisher Cannon pieplate on them only for their 4+ invuln save to shrug it off is hilarious, and they're likely to keep anti-tarpit SCs like Lelith Hesperax or Typhus away because of the Force Weapon used by the Sorcerer.
• Rhinos - OUR ENEMIES HIDE BEHIND METAL BAWKSES! THE COWARDS! THE FOOLS! We... We should hide behind our own metal boxes... Has exactly the same advantages as the block of cover used by the loyalist bastards, with a Havoc and some other fun stuff thrown in.
• Plague Zombies - Now troop choices in CSM armies by taking Typhus as a HQ. Take 35. They're both fearless and feel no pain and a massive pain in the arse for anyone planning on shooting anything.

Sisters of Battle

• Ecclesiarchy Battle Conclave - Storm shields for 15 points? Yes please. Don't expect them to survive against TEQs for more than 2 turns though.
• Blobs of 20 sister of battle with flamers and priest. Fearless, hainous, rerollable 3+ save wall of flame for the same price as marinescouts. The true meaning of tarpit

Lost and the Damned

• Mutants - This unit basically exists for this purpose.
• Plague Zombies - World's best tarpit. Cheap as Conscripts, and slow moving, but has feel no pain and fuckhuge numbers. Just don't let the tau see them or they vaporize.
• Traitors - See also: Guardsmen.