Discussion Forums

comp.graphics.apps.photoshop - a chaotic mix of good people and idiots - typical usenet. Extremely active.
comp.publish.prepress - highly technical, but a source of info you won't find anywhere else. Lots of bitter prepress people sharing horror stories about clueless designers. Worth skimming for most designers, but don't feel bad if the discussions of Postscript code or the intricacies of oil mounting go over your head.
alt.graphics.illustrator - last time I checked there weren't enough savvy regulars to make this a must-read, but hey, times change.
comp.fonts - a few professionals make their living off of sales generated by the good advice they dispense on this group. Just don't post asking "can anyone email me a copy of Adobe Futura?" or you'll get blacklisted - people who make their living selling copyrighted fonts don't take kindly to such behavior.