Mod-hq is reporting that the Half-Life 2 expansion "Aftermath" will be released on April 24th. Doug Lombardi of Valve confirmed this report himself. Also released on April 24th will be a "Platinum" package for Half-Life 2, which will include the following: Half-Life 2, Half-Life 2 DeathMatch, Counter-Strike: Source, Day of Defeat: Source and of course Aftermath.
Half-Life 2: Aftermath is an expansion to the multiple game of the year winner Half-Life 2. The game is set ironically right after the end of Half-Life 2, where you have dealt a serious blow to the Combine, and the Citadel itself is exploding around you. Gordon Freeman, and Alyx will both be playable. City 17 itself will of course have changed in the expansion, monsters previously off limits will now appear. Not much else is known about the storyline, but being a Valve game, we can be sure it will be a blast.
*Update 1/11*
Teamkilled.net has now confirmed that Valve will re-release a version of Half-Life 1 DM running on the Source engine with Aftermath and that the retail price will be around $20US.