Valve released Portal, a new game using the Half-Life 2 game engine.
This game is loads of fun, you can really do a lot with the portals...defy gravity, fling yourself across vast distances, overcome traps and puzzles. Best of all, the game's not too difficult; even the timed areas give you more than a few moments to assess the situation.
The only problem I had was with the commentary. Once it's on, you don't earn any more accolades (fun rewards)...and I can't figure out how to turn it off!