KvrAudio contains loads of audio plugins, both free and commercial, and great community forums.
Vst Cafe is a blog about good, cheap, free stuff.
If you like game music and want MIDI files, there is a huge collection here: http://www.vgmusic.com/
Over time some threads on the Forum also represent a useful resource. This one has a lot of interesting links found by LMMS users: Users-choices