The fuck is Synergy? All I have is "Half Life 2: Deathmatch". I think akunt does too.
Synergy is like deathmatch, but it's free and you can play single player missions with a team, you can give health packs, weapons, and ammo to others as well. And most of the time custom vehicles spawn if you need to drive and there isn't enough seats. And some more things.
Quick Summary:
It's free in steam. It's fully supported with a dedicated server and shit. It's basicly deathmatch but working together in singleplayer with a few fixes.