Synergy is a Half-Life 2 co-opertive multiplayer modification. We have evolved from multiple mods (DC Co-op 2, Dev Co-op and Tim-Coop) to offer you the greatest co-op experience. This modification strives to give Half-Life players a true Half-Life 2 cooperative experience. The mod features our own levels and the ability to play the Half-Life 2, Episode One and Episode Two campaigns co-operatively. In addition, you can add more content by installing third-party mods, such as ROCK 24 and MINERVA: Metastasis, then playing them in Synergy!
Not badly designed, but the mod has crashed on me a few times, and getting a good server can be difficult at times.
Also -- Half-Life 2 levels are designed to be challenging for 1 player. Playing with more than 4 breaks almost all the difficulty. I suggest getting additional maps to play on, or you'll be completing the Half-Life 2 campaign faster than you can say "Alien Swarm".