Book and careers forum. I run this one to help readers and job-hunters.www.cqf.info smaller but friendly and pleasant
www.global-derivatives.com hmm, well, another one
www.moneyscience.com a bit of a different emphasis: they are a financial services directory. I do training courses with them.
www.nuclearphynance.com more oriented towards trading, slightly aggressive atmosphere for better or worse, and operates on a non-commercial basis. Some participants really know their stuff.
www.quantfinancejob.com very new and aimed at getting a job (features some good reviews of my books!)
www.wilmott.com a big forum which exists to market Wilmott magazine, CQF training courses, and the paulanddominic headhunting outfit.