Half-Life 2 - An FPS, regarded by some as the best game ever created
HL2: Episode One - the continuation of HL2
HL2: Episode Two - the continuation of HL2: Episode One
Portal - A Brilliantly fun, and inovative FPS Puzzle game
Team Fortress 2 - A cartoony and funny online multiplayer FPS
You also get 2 extras, which you can download from Steam for free after The Orange Box is installed:
HL2: Lost Coast - an extra level for HL2
Peggle Extreme - a Orange Box themed version of the classic PopCap game Peggle Deluxe
If you are a gamer, this is too much of a good deal to pass up.
*WARNING* this game requires Steam. It's hard to explain exactly what steam is, so look it up if you don't know.