"myriad" (official definition: 10000) is a bit of an exaggeration ;-)
The N900 is a bit poorer in that respect. Options for previous devices include Mamona, Mer, Deblet, Poky, Gentoo, Ubuntu, OpenWrt, and probably a few others I forget. It also depends on your definition of "other OS", if you include things that run on top of Maemo there's also the various easy-* chroot environments and emulators like Garnet, DOSbox etc. Edit: there's also some info on the wiki: alternative operating systems, alternative desktop environments, emulators.