The Source SDK is freely available to all owners of Half-Life 2. At this point, no other Source game gives access.
Downloading and Installation
To install the Source SDK:
- Open the Steam window.
- Click on the Tools tab.
- Double-click on the Source SDK entry.
- The Source SDK will then be downloaded to your computer via Steam.
Once it has been fully installed, you will be able to launch it in the same way you installed it (from the Tools tab).
You can also create a desktop shortcut for the Source SDK by right-clicking on the Source SDK entry and clicking Create desktop shortcut from the context menu.
Where do I go from here?
The main page of the Source SDK documentation is a good place to start.