The Quantitative Finance Stack Exchange community entered public beta last week. To quote the FAQ:
The Quantitative Finance Stack Exchange is intended specifically for professionals and traders working in investment banking, and academics involved in teaching and research.
Topics include pricing of securities, derivatives, options, risk modeling, quantitative techniques, and the mathematics used in quantitative finance.Several members of the R/Finance community are active there too. Some names you may recognize: Patrick Burns, Shane, Dirk Eddelbuettel, Jeff Ryan, and Paul Teetor.