One of the best FPSs of all time. Just play it.

For 4 player local co-op, you can't go past Broforce. Controllers are required for four local players, but Steam online play is also built in.

Great for 4 player local co-op, if you have enough controllers handy. Great fun, and just difficult enough to keep you playing.

Halo-inspired FPS, with graphics that were never meant to look next-gen. You can play as a zombie with tentacles.

Probably the best shooter available for LAN parties.

With support for up to four players, Magicka is a relatively linear RPG, where you cast your own spells using elements, most of which can kill you.

A small, relatively cheap game, with online rankings that allow you to compete in your own time. Insanely addictive.

A hugely popular turn-based strategy game. Time consuming, but surprisingly historically accurate, this is a great game for long online matches.

Ideal for LAN parties, AoE2 has been rebuilt in HD with Steam multiplayer integration.

Being able to play with others while not at a LAN party is important. DotA 2 is the most popular game on Steam, and the world's highest rated MOBA.