On linux you can use SM via vmware/wine and on os x it's quite usable with parallels. If you're using SRS just for language learning then being able to use on mobile can be helpful but I think for those cases supermemo.com can be a decent alternative. More generally, I think the platform specificness of SuperMemo is more than made up for with its incremental reading functionality (alongside better scheduling algorithm)
To add to this, there's a set of Winetricks recipes for running SM on Linux.[0] (Disclaimer: I haven't tried them, though I've been planning to, for a while.)
