I found these addons which can improve the usability of Source SDK. They can add things like Python Support, additional tools, improved compatibility, etc. Please note: Image is just a placeholder.
Posted by AmaroqDricaldari on Apr 17th, 2011
I put these here as they can help both the Source Community and the smaller Annihilation community. Perhaps when this engine is finally released, some of these tools will be included.
Here are some of the ones I found:
And here are some more:
I will add some more links to specific tools when I find ones that will be particularly useful to the Source Community.
Note: The Annihilation Engine is a community Addon for the Source Engine.