Roughly ordered by functional (not skill level) simplicity, that is they go from simple (basic scripting) to complex (full Photoshop Extensions).
- Photoshop Script reference - Start here for all your scripting needs. Check out the script listener plugin too. It outputs anything you do in Photoshop as Javascript.
- Adobe Configurator - Build quick (but simple) panels. Basically GUI interface for scripts.
- Adobe Photoshop Scripting and SDK forums. Somewhat helpful for scripting/extension building. Don't expect quick answers.
- PS-Scripts - Another semi-helpful forum
- Photoshop CS5 Panel Developer's Guide - Hard to find now. I don't think they made one for CS6, but it's still releveant for some things.
- Flash Builder - What I use to build GuideGuide.
- Adobe Extension Builder - Because using command line tools is painful
- Photoshop SDK - You'll need this if you want to build Photoshop extensions.
Here's some people that are active in the Photoshop development community. They tweet useful tips, have good knowledge, make cool things. They're cool people, so please don't harass them.