Biggest and best is probably XDA.
http://forum.xda-developers.com
You can find a giant list of devices each with it's own active sub community. Roms and software are usually found in the "development" sub forums. Be sure you use the search tool because some times solutions to problems are sub sets of larger or different problems. You'll also get a negative reaction from most folks if you don't say/show that you made a best effort to think for your self before posing your question.
Most comprehensive general forum seems to be Android Forums.
http://androidforums.com/
These forums seem to be slightly less focused on development and more on general usage. They do have their fair share of brilliant folks. Being less specifically development focused they seem to have support for a larger variety of devices.
Getting gradually less technical but still useful threads and information at Android Central's forums.
http://forum.androidcentral.com/
Android Central is a pretty well respected Android news website and community. Their forums are less technical than the other two but they still have a great group of people willing to help and some good info to learn from.
Finally the Nook Color specific site Nook Devs.
http://www.nookdevs.com
This site is Nook specific but has some pretty clear concise and useful information and how-to on general Android things like using ADB. It's also the place to go on information about rooting the Nook and Nook Color.
So here's the breakdown on how Android is structured, as I understand it. I think I'm pretty astute and seem to have little trouble rooting/flashing and fixing things gone wrong on Android but I'm also not perfect so I could have some things wrong. So if you know more or see something I got wrong please let me know in the comments!

