A project that I checked out a looong time ago has come a long way since then (well, my studio setup has also come a long way since then) that I think is really cool and a great complement to LS: Bristol. The synthesizer emulator for GNU/Linux.
Moog, Oberheimer, Prophet, ARP, Roland, Yamaha, Fender Rhodes, Vox Continental, Hammond!
(Note that they're not exact emulations, but they're fun to play with.)
Wikipedia (en) a.k.a. "the missing manual".