If you are a fan of the Half Life series, you probably have been eagerly anticipating the release of the Half Life 2 expansion, "Episode One". As of yesterday, Valve has now released it! Anyone using a Steam client can already receive it, and for the cost of $19.95 it can be unlocked and played. It's also available in stores. Episode one is a standalone game that does not require the original Half Life 2 to run, although it uses the same game engine and many of the same elements. They have a teaser up if you want a preview, though you may want to instead stick to the various game reviews that are already starting to surface.
On a very interesting side note, less than a day after its official release, Valve made a patch available that fixed a minor quirk in the menu system. If you liked Half Life 2, odds are you will like this. According to Valve, they will be releasing yet another addition to the HL universe, "Episode Two", later this year.