The Orange Box Achievements

The Orange Box for the Xbox 360 has the most achievements of any Xbox 360 game with 99 achievements (However it only has 1000 possible gamer points, Halo 3 and Gears of War 2 tie for the most gamer points in a single Xbox 360 game.)

On its Xbox 360 release, The Orange Box received 99 achievements across all 5 games on the disk. On their release, the Steam versions of Half-Life 2: Episode Two, Portal and Team Fortress 2 all saw achievements. Until recently, for Half-Life 2 and Half-Life 2: Episode One, all achievements were exclusive to the Xbox 360; However, in the first days of the Steam UI Beta, all of the Half-Life 2 and Half-Life 2: Episode One achievements were shown on the game's Library/Games page. Users speculated that Valve would include those achievements for the PC, which they did on 26th of May 2010. Note: The Team Fortress 2 achievements are not detailed here.

Any Half-Life 2 game
There are 6 achievements worth a total of 40 gamerpoints shared by the three Half-Life 2 games in The Orange Box. These achievements were originally only in Episode Two on PC. All six are now attainable through Half-Life 2 alone, though they may also be unlocked in Episode One and Episode Two.

Half-Life 2
Half-Life 2 has 32 achievements with 275 gamerpoints unique to it.

Episode One
Half-Life 2: Episode One has 12 unique achievements worth 135 gamerpoints.

Episode Two
There are 19 achievements in Half-Life 2: Episode Two worth 210 gamerpoints.

Portal
Portal has 14 achievements worth 175 gamerpoints and an additional achievement only available on Steam versions of the game, that was a part of the game's ARG.