Rapture

Couple of things for the prospective modder to check out:

GDM, a journal for the pros, interesting outlooks inside

The Valve Developer Wiki, basically the handbook for using Valve's editing tools. Infinite knowledge is stored here, it explains every keyvalue of every entity in the game. However if you want more in depth explanations and examples of implementation, you'll have to head over to some sick nasty...

...Tutorials. My favorite are the SDKnut's tutorials, which explain most of what you would want to do. It helps to have a human guiding you step by step sometimes. Which is why the next one is so useful.

Forums. The official Steam community forums have years of questions asked and answered and if the archives don't help you it's likely that an active member can.

The Mod database.
Here you can view the lives of nearly every single Mod out there. There lies hints to what makes a mod successful, as well as tips to get over the slump a lot of mods hit halfway through their production.

Not sure how useful this will be to anyone but me, but I think if someone had helped me out with this information I would have had an easier time getting acclimated.