Venomthrope
What the Zoanthrope is to the concept of "psyker", the Venomthrope is to the concept of "poison". Floating above the ground on organic balloons full of toxins and sporting enough tentacles to make Cthulhu blush, the Venomthrope is effectively a short-range terraformer for the Tyranid invasion force. The cloud of spores and bacteria it emits are not only a comfortable little bubble for Tyranid units, they're a huge pain in the ass for any of those pesky planetary defenders that might have survived this long. It relates to the Toxicrene in roughly the same way as the Zoanthrope does to the Maleceptor, except that the Toxicrene does not suck.
On the Tabletop
The Venomthrope has 2+ poisoned attacks and gives Shrouded to all units within a 6" inch radius. This makes them extremely useful as support for your masses of gaunts so that they have at least some kind of save. This also works great for supporting your Carnifexes and Hive Tyrants alongside their followers: that cover save is a blessing when dealing with AP3 weapons. This obviously makes the Venomthrope a premier target for the enemy forces: especially Tau are laughing as they ignore your cover save and one-shot your entire brood with a powerful weapon. Template weapons are dangerous as well: a few shots with the equivalent of a Heavy Flamer will do a lot of damage on your Venomthropes. But as long as you keep them in the back and protected by units that can devour whatever unit that comes near it, discouraging suicide tactics a bit. And make sure you take out the opposing artillery ASAP.
8th Edition
Venomthrope still has a nice benefit: enemies that shoot at shrouded tyranids suffer -1 to their Ballistic Skill. This is NOT ignored by anything that Ignores Cover, cover in this edition applies +1 to saving throws instead. Although flamers still auto-hit, so there's that to worry about. Now require a brood of at least 3 if you want the sporecloud to affect your monsters, and a full brood of 6 extends its cloud to a range of 9". Overall these cuties are now less useful than they were in 7th edition and are generally outshined by the Malanthrope, but they're still playable.