The latest edition of the Source software developers kit adds support for the three most recent releases from Valve in Portal, Team Fortress 2 and Half-Life 2: Episode Two. You may know them collectively better as The Orange Box. That means we should be on the receiving end of a slew of new Portal and Team Fortress 2 user created maps courtesy of the Hammer World Editor and fabulous machinima nearing the levels of Valve's internally created TF2 character class clips courtesy of the Faceposer tool.
Details on the Source SDK are available at the Valve Developer Community wiki and should give budding map designers a good place to start. Quick, someone remake a working de_dust2 in Portal before the next guy does.