Jam Studio is an online version of GarageBand. It lets you create your own song backings, by mixing 8 clips, each of one bar, from a library of "40,000". I find it even easier to use than GarageBand, as it includes a section structure, reminiscent of tracker's arrangements, so that you can, say, write the chorus once, and then very easily reuse it throughout the song.

The deal-breaker limitation is that it offers only bar-level sequencing: There is no way to create new melodies. Still, for making backing tracks, this is the easiest method I've ever seen.