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 Millenium 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 - 20+ Models for dirt cheap. Do the math.
• Regular Guardsmen - While not as cheap as conscripts, they can still make efficient tarpit units if you send out your non-kitted guys to charge someone.

Chaos Daemons

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

Inquisition

• Inducted Guardsman Squad - still not expensive.

Tau

• Kroot Carnivores - Inexpensive infiltrating models at 7 points each. Hell of an area denial unit.
• Gue'vesa Auxillaries - Even less expensive and can't infiltrate but shooty and with decent wargear options.

Tyranids

• Ripper Swarm - vulnerable to Instant Death and blast templates, but will tie foes up for round after round if they get close.
• 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...etc).
• Gretchin - Dies like bitches to nearly everything, but numerous, can hold objectives, dirt-cheap, and can actually shoot better than most Ork boyz.
• 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, can get a 2+ save with turbo and a fuckton of wounds, unless you get attacked with ID or blast templates.
• 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 stunlocking vehicles into oblivion or destroying them if you're lucky.

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. They're better off shooting than knifing someone to death though, unless you use Storm Guardians.

Dark Eldar

• Grotesques - While not really a tarpit and they're not really a good take in games since they hog up a precious elite slot, they're mad tough and if still led by a Haemonculus, could tie up something powerful like termi squads in a turn or two before they either tard out or die miserably. Against weak infantry units, they could probably tie them up for atleast half the game or kill them outright.

Marines

Factions like the Space Marines, Chaos Space Marines and Grey Knights do not to have any real tarpits, due to the fact that most of their infantry squads are small and they're all fairly expensive compared to other factions. Space Marines, however, can use Scouts (which can infiltrate), Space Wolves get Fenrisian Wolves, and a properly kitted-out Grey Knights army can flood the board with inquisitorial henchmen. Back before the current edition wrecked their shit, Chaos Marines could flood Cultist Squads for this purpose. Nowadays Plague Marines make surprisingly effective tarpits, mostly thanks to T5 and Feel No Pain, which makes them goddamn frustrating to kill.