Clearly, these are the best forums out there. But if you want to have a look at some others, try AppleInsider, ThinkSecret, Mac OS X Hints, and MacOSX.com. There are others (including Apple's own support forums), but those are the ones that immediately jump to mind.
WildCowboy Mentioned all the big ones. However I would have to say I have tried most of them and macrumors has the fastest response time and also has lots of great people. After all we have Mad Jew he is always on helping people. Have you repaired your permissions lately?
The MacNN forums are sometimes said to be the largest Mac forums around. In terms of number of registered users, MacRumors is a lot bigger. However, MacNN has more posts and threads than MR. I joined both about the same time, and visit both regularly, but spend most of my time here, where my post count is about 20 times as high.