My list of add-ons probably worth recognizing:
Team Deathmatch
C4 Explosive (Plornt)
GSF maps and vehicles
JVS
RTB v3.3
Parachute
Slate Edits
Tier+Tactical
Default Minigame
All these,
Ion Cannon, I never hosted online play, so I used it in single player
(and I didn't crash when I used it on saves of over 9000 bricks :D),
Mini-Nuke, same as Ion Cannon,
AH-1 Cobra,
Ai_national socialist,
Gamemode_Zombie (I can't believe this has not been posted),
Heedicalkings Modern War pack,
and the new L4D pack, even though I know that it was Bushido and Yndaaa that made them, not Rykuta and Yndaaa.