You can download the latest released version (v1.0.1) of the Elysium sequencer for Mac OS X here. This version was for Snow Leopard (10.6) and was known to have problems on Leopard (10.5). It has not been tested on Lion (10.7) so feedback about that is welcome.
Elysium was a first step into making music. Having no musical background at all when I saw a video of the reacTogon my reaction was "Here is an instrument I could play!". Realising I'd probably never meet an actual reacTogon I decided to try (with the blessing of Mark, the creator of reacTogon) and make one in software. Elysium was the result.