Found an article over on Bluesnews which touches on the modifications that will be available (sometime) with HL2

.Q: Will mods be set up the same way as they are in Half-Life 1 (will there be folders for each mod)?
A: Yes, they will be set up the same way as Half-Life 1.
Q: Can maps be as big as they are in Battlefield 1942 and if not, can they be made to be larger?
A: The maps in Half-Life 2 are much too dense in objects to make them as big as BF1942. The ability is there but you wouldn't be able to make them with as many objects as most Half-Life 2 maps are, and if you do, you would need a insane amount of RAM to handle it.
Q: Team Fortress Classic, any news on it going to source or TF2?
A: TF2 is being worked on at VALVe, and will most likely be announced after Half-Life 2.
Q: Day Of Defeat: Source, how about any news about it?
A: Day Of Defeat: Source is being worked on but will not be available at Half-life 2 launch.
Q: Will the new level design allow Terrain Deformation, or will it be be more like Half-Life 1 (where you had to set a breakable spot in Hammer)?
A: Half-Life 2 will not have any Terrain Deformation, but it can be enabled or used in mods. On a side note, Team Fortress 2 will have Terrain Deformation.

A few good mods on the way as it would seem then.
Full interview