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 so