I like StackExchange sites. My favorites stackoverflow.com and programmers.stackexchange.com
You can chat with people there, discuss the white-board questions etc
IRC channels for projects can be quite lively. The busiest channels I idle in are #ffmpeg-devel and #lua on freenode. Clearly these are dedicated towards these projects but sometimes other chat filters in. #lua certainly gets that.
Others can be much more quiet and be usually for tech support of a project but they can still be a good place to contact people working on a project.