Try this list and check if anything there fulfills your fancy.
http://en.wikipedia.org/wiki/List_of...and_sequencers
My personal tries:
Hydrogen seems to be an alternative to Fruityloops.
http://www.hydrogen-music.org/
There's also Skale, but even more confusing than FL
http://skale.binarydeception.com/
The closest you can get to FL is GNU/Linux exclusive, too bad. Only closer is ABLETON.
http://muse-sequencer.org/