This site -- DNForum.com -- is a clearing house for discussions on a variety of topics on domain name sales, domain appraisals, and discussions about domain registrars.
Their "Industry Leaders" forum allows you to talk about issues with registrars. If you're like me and have a lot of domain names, then you've likely run afoul of some registrar at some point and this is a great place to get tips on how to deal with individual registrar issues.
Possibly the best opportunity on this site though is if you have some domain names you want to sell. Here you can get an appraisal and then create buzz and network to sell that domain you've been holding on to.