Perpetual Motion is probably the oldest prog/power metal message board on the net. It was original run by one guy, then he got burned out and another guy took over. Then the first guy came back with a new forum, and the new guy kept his going, calling it PM:X. Then PM:X died, and now Perpetual Motion is all that remains. The fragmentation of the community that occurred when there were 2 boards has left the traffic somewhat diminished, but they still keep chugging along.
After the original Perpetual Motion board died, a new one was built in its stead. Later, the original board was reformed and the new board was re-dubbed PM:X. That board kind of died off after a while because of hosting problems, and the new PM:X board arose.
While the original board caters more to fans of Dream Theater clones and power metal bands, PM:X seems to cater more to extreme metal fans.
Ultimate Metal began as a small online review site, but quickly expanded through hosting various band and other metal sites' message boards. It is now the largest online metal community that I know of.
Librarius Metallicus hosts its forum there, as well.