While I haven't used it directly for anything serious, there's Ardour.
I'd recommend loading up a machine with Ubuntu Studio or similar (AVLinux, KX Studio, etc) and see what you can do. It'll cost you nothing. Ubuntu Studio comes with Ardour and quite a few plugins.
On the iPad as well as on a Linux PC I prefer software similar to Ardour, e.g. Musicstudio, Qtractor, Rosegarden, however, taking a look at the free version of Garage Band, it seems to follow a completely different approach. Perhaps no Linux software comes close to Garage Band, but "a next step" might be taking a look at LMMS too.
LMMS has got another approach than Qtractor, Rosegarden, Ardour etc. and some users seem to prefer this workflow.