http://storefront.steampowered.com/v...ch&category=16
Mind that the free HL2DM which owners of ATI cards get doesn't contain the Source SDK, however if you buy it via Steam, it's included.
While these games come with the Source SDK, you only need to install Source SDK Base. Difference:
Source SDK is for
creating maps, models, mods etc,
Source SDK Base is needed to
play mods.