The game can be played many different ways and allows for a multitude of different amount of players.

Co-op FPS gameplay against monsters.

Fast classic FPS deathmatch. One of the originals.

Extremely moddable. Sven co-op with a combination of other mods can create single-player experiences that everyone can join in on.

Using Sven Co-op mod you can create the Half-life experience that everyone can share.

Updated version of Counter-strike: Source. Involves more weapons and maps.

Classic team-based LAN gaming. Free.

Just like the first one with more variety in gameplay.

Classic 4 player co-op or 4v4 versus with a more polished gameplay than the sequel.

A good online 4-player game with a mix of old school mechanics and new gameplay mechanics. Can be competitive.