Valve has announced a number of additions that will be appearing to download in Steam pretty soon, the most exciting of which being a new single-player level. "The Lost Coast" will be free to download and will include High Dynamic Range lighting (HDR). This will only be available on high-end video cards, but the game will detect if you have the hardware to support it, disabling it for owners of less cutting edge cards.
CS: Source players will be pleased to here that cs_compound is going to be available soon and the crowbar, Stunstick, and Slam will be added to HL2 deathmatch. A new deathmatch map will also be added while Valve judge the map making contest.
All in all, not a bad lot of updates to an already great game. We'll bring you more news as it becomes available.