Half-Life, Half-Life: Blueshift, Half-life: Opposing Force, Half-life: High Definition pack (might not classify as a game), Half-Life: Source, Half-Life 2, Half-Life 2: Episode 1, and Half-Life 2: Episode 2. Eight games total not including countless mods created by fans that you can download for free on moddb.com and fileplanet.com. Other games by Valve software include Left 4 Dead, Left 4 Dead 2, Portal, Portal 2, Counter Strike, Counter Strike: Source, Team Fortress, Team Fortress 2, and Alien Swarm.

Another Half-Life game would be Half-Life: Decay. You might also be able to count Half-Life Deathmatch and Half-Life 2 Deathmatch.

you start with half life 2, then you play half life 2 episode 1, then you play half life 2 episode 2, so you are basically playing them in the order they are in at the game select screen (where you choose any of the half life games, portal or tf2)

No, only Half-Life and Half-Life: Decay.

Half Life 2

Half Life 2: Episode 1

Half Life 2: Episode 2

Portal

Team Fortress

Hope this helps ;)

Buy Half Life: Source Or Buy Garrysmod a Half-Life 2 Modification and get the addon "Half-Life Rennaisance" Link to Half Life Rennaisance: http://www.garrysmod.org/downloads/?a=view&id=57033 Enjoy!