Piranha
The Piranha (they live in the Amazon) is a light two-seater skimmer (and the best in kamikaze skimmer technology) used by the Tau Empire. It began life as an unarmed low-cost ground transport for commanders and other important personnel, much like the Imperial Guard's Salamander Reconnaissance Tank, but during the Taros campaign it was given some extra armor and weapons to serve on the battlefield.
It comes armed with a burst cannon and a pair of gun drones, for mopping up most infantry, though it can swap the burst cannon for a fusion blaster and pack a pair of seeker missiles for hunting vehicles.
Its speed means that it is usually tasked with reconnaissance, though they are also used in rapid-response forces and security teams (loaded for whatever targets they expect to encounter). Though it is not tough enough to take a shot from a tank, it can weather infantry weapons and it is agile enough to harass much heavier vehicles until armor support can arrive.
Piranhas have the ability to bring firepower quickly to wherever it is needed, but they are even more dangerous when working in conjunction with Pathfinder Teams. Using Marker Lights, Pathfinders often set up Piranha strafing runs, ensuring that the skimmers' volley of fire is as devastating as possible. In return, Pathfinders that find themselves in danger of being overrun can request quick strikes by Piranha teams to help wipe out approaching enemies or, at the least, buy time for the Pathfinders to reposition.
The model was first made by Forge World for Imperial Armour Volume Three: The Taros Campaign, as an in-universe field test, and was eventually brought into the fourth edition Tau Empire Codex.
Piranha TX-42
The TX-42 is the newest incarnation of the original Piranha, intended to fulfill the role of a heavy gunship designed for frontline operations in support of Tau armored units instead of a lightly armed reconnaissance craft.
The TX-42 is a slightly heavier variant of the Piranha more suited for hunting vehicles or heavy infantry -- it has no gun drones, but comes with twin-linked fusion blasters by default, with the option to take twin-linked missile pods, plasma rifles, or rail rifles. It also has the same vehicle upgrades available to the standard Piranha variant.
The TX-42 also features a reworked crew compartment, providing much greater protection to the two-man crew enclosed within. Although the compartment remains open, the increased armor allows the TX-42 to brave weapons fire that would have quickly destroyed the Piranha, allowing it to deliver pinpoint heavy weapons strikes in even the heaviest fighting.
Still in the prototype stage of development, the TX-42 has yet to be mass-produced. In fact, it has so far been reported in use only amongst forces originating from the Sa'cea Sept. These forces are expanding through the Perdus Rift region as part of the Tau's Third Sphere Expansion and appear to be conducting tests of this new vehicle in combat conditions.
Should these tests prove successful, it is likely the design will quickly spread to Tau cadres across the Empire.
Bad news, it's no longer in production. Thanks Geedubs.
Tactics
The T'au Codex says you can take up to 4 in a squadron. Logic says DON'T. While 4 speedy buggers may seem useful, the reality is if you lose too many, you very well may lose the rest. Why? They only have Leadership 6, and do not have the Bonding knife ritual special rule, making them very vulnerable to bad leadership rolls.
Where they shine though, is in their fluff described duties. At Toughness 5 and with a 4+ save, in addition to the 6 wounds, you will find turning GEQs into mulch will be a piece of cake, while easily being able to whittle down those pesky MEQs and TEQs. Also important: they don't lose any of these characteristics, no matter how many wounds you lose on them. Additionally, they can take two Seeker missiles per model, allowing you to use them as small artillery platforms for those extra wounds on a distant target.
Combined with the Vior'la Sept Tenet, these things become absolute beasts, able to advance further than their already high 16" of movement, just so they can get close to an enemy tank, infantry squad, or vehicle, and unleash holy dakka upon them. The Two Gun drones are kinda stuck with you, but at the advantage of being a pest to harass tanks and infantry, and can lock down objectives while your other infantry proceed to other targets. So really, the only question is what you want your Piranha to do. Hunting tanks? Go with the Seekers and Fusion. Killing ungodly hordes of Infantry? Go burst cannon.