Synergy - A cooperative mod. Synergy supports official campaigns by Valve, user-made levels, and other third-party mods (Minerva and Riot Act so far). A major update was released on Steam, using the Orange Box engine for new features.

Hidden: Source - Pits one player (called The Hidden) as an invisible genetically-modified human against a team of SWAT-like operatives called the Infinitum Research Intercept Squad, (IRIS) with regular human attributes but a selection of weapons to choose. The IRIS team's objective is to hunt down the Hidden, while the Hidden attempts to stalk and kill the IRIS team without being detected.

BattleGrounds 2- A mod that takes place during the Revolutionary War. (In my opinion it is an excellent mod.)

Zombie Panic! Source - A team-based zombie themed mod. Zombie Panic! Source pits player-controlled survivors against player-controlled zombies, in a gameplay style that simulates an outbreak. The zombie team starts with only a few players, but their ranks grow as survivors are mauled and switch to the zombie team

If I think of or find anymore I shall let you know...