Your list
Moth_Monk said:
Half Life - 1998
Team Fortress Classic - 1999
Half Life Opposing Force - 1999
Deathmatch Classic - 2000
Ricochet - 2000
Counter Strike - 2000
Half Life Blue Shift - 2001
Day of Defeat - 2003
CS: Condition Zero - 2004
Half Life Source - 2004
CS: Source - 2004
Half Life 2 - 2004
Half Life 2 Deathmatch - 2004
Half Life Deathmatch Source - 2005
Day of Defeat Source - 2005
Half Life 2 Episode 1 - 2006
Half Life 2 Episode 2 - 2007
Portal - 2007
Team Fortress 2 - 2007
Left 4 Dead - 2008
Left 4 Dead 2 - 2009
Alien Swarm - 2010
Portal 2 - 2011
Dota 2 - 2012
CS: Global Offensive - 2012
Half Life 3 - 2098
Now let me tell you how many are considered games, just by Wikipedia, I'll quote wiki each time
Half Life, 1998 - "Half-Life is a science fiction first-person shooter video game" Game:1
Team Fortress Classic, 1999 - "Team Fortress Classic was originally released for Windows on April 7, 1999 as a modification to Half-Life." Game:1 Mod:1
Half Life Opposing Force, 1999 - "Half-Life: Opposing Force is an expansion pack for Valve Software's science fiction first-person shooter video game Half-Life." Game:1 EP:1 Mod:1
Deathmatch Classic, 2000 - "DMC is a multiplayer video game developed by Valve as a tribute to Quake." Game:2 EP:1 Mod:1
Ricochet, 2000 - "Ricochet is a multiplayer mod of the popular first-person shooter Half-Life" Game:2 EP:1 Mod:2
Counter Strike, 2000 - "Counter-Strike...is a tactical first-person shooter video game" Game:3 EP:1 Mod:2
Half Life Blue Shift, 2001 - "Half-Life: Blue Shift is an expansion pack for Valve Software's science fiction first-person shooter video game Half-Life" Game:3 EP:2 Mod:2
Day of Defeat, 2003 - "Day of Defeat is a team-based multiplayer World War II first-person shooter video game" Game:4 EP:2 Mod:2
CS: Condition Zero, 2004 - "Counter-Strike: Condition Zero...is a multiplayer video game" Game:5 EP:2 Mod:2
Half Life Source, 2004 - "Half-Life: Source is a straight port"? Game:6 EP:2 Mod:2
CS: Source, 2004 - "Counter-Strike: Source...is an FPS video game" Game:7 EP:2 Mod:2
Half Life 2, 2004 - "Half-Life 2...is a first-person shooter video game" Game:8 EP:2 Mod:2
Half Life 2 Deathmatch, 2004 - "Half-Life 2: Deathmatch is a multiplayer first-person shooter video game" Game:9 EP:2 Mod:2
Half Life Deathmatch Source, 2005 - "You may create the page "Half-Life Deathmatch Source", but consider checking the search results below to see whether the topic is already covered." Game:9 EP:2 Mod:3
Day of Defeat Source, 2005 - "Day of Defeat: Source is a team-based first-person shooter multiplayer video game" Game:10 EP:2 Mod:3
Half Life 2 Episode 1 - 2006 *Gives up quoting*
Half Life 2 Episode 2 - 2007
Portal - 2007 Game:11 EP:4 Mod:3
Team Fortress 2 - 2007 Game:12 EP:4 Mod:3
Left 4 Dead - 2008 Game:14 EP:4 Mod:3
Left 4 Dead 2 - 2009 Game:15 EP:4 Mod:3
Alien Swarm - 2010 Game:16 EP:4 Mod:3
Portal 2 - 2011 Game:17 EP:4 Mod:3
Dota 2 - 2012 Game:17 EP:4 Mod:3 (not released)
CS: Global Offensive - 2012 Game:17 EP:4 Mod:3 (not released)
Wow, more than I expected, I wouldn't consider Counter-Strike (except CZ) or Team Fortress to be self deserving games, but Wiki does so I'll leave it at that, ~17 with them, otherwise I'd only count 5 as games, and 11 of those games as Mods/EPS.
So really, its HL, HL2, Portal, Portal 2...Alien Swarm... The rest are all derivitive/remakes/ports/expansions, ect.