1. Tactics trainer
2. If there's a specific skill you want to work on and there's a group for that, check it out
3. The opening book is nice for studying openings
4. Play lots of games :)
5. Ask some people to analyze your games (I personally go around to different forums or people inside my groups, or friends, and analyze their games when I have some time to kill).
Hope that helps!
I've known the basic moves of chess for many, many years and I learned one opening that I basically always use from about 10 years ago. I never really got into studying, learning and playing much. I'm new to this site and basically would like to know, where is a good place to start to develope my chess playing skills?