The Freenode and Efnet IRC networks have reasonably active math/physics/chemistry/neuroscience/... channels, with some of the channel regulars being experts in the area (often grad students and faculty). We've been trying to keep the Freenode ##physics channel in a decent shape for the last several years, but YMMV, depending on the area of interest, the time of day and random chance. More often than not, people just drop by for homework questions, but there are also plenty of more general discussions. The downside is that very little of it is posted online.