Great source mod, you need episode 2 on steam to play it, though.
It pretty much just adds monsters, new levels, and leveling up to the game. (If you do play, go on the Nachos and Cookies server, they give more level up points then normal and I always play there.)
It's been seriously modified since a few months back, they took away most tank abilities due to too many tanks, I guess. I must admit that it used to be much more fun than it is now, but it is still great.
There's three factions (Aperture, Combine, Rebel). On team based games, when team mates are close to each other they get bonuses, that are different per faction.
There' four game modes (Players versus Monsters, Team Deathmatch, Deathmatch, Free for All)
Players versus Monsters is pretty much the core gamemode. The three factions fight monsters and can't hurt eachother, but the faction with the highest average exp per player gets bonus exp for all their players.
DeathMatch is basically just deathmatch with leveling up.
Team Deathmatch is all three factions fighting eachother. Faction with highest average exp per player wins.
Free For All is deathmatch with monsters as well. This can be fun at times, chaotic at others.http://www.moddb.com/mods/modular-combat