Coda runs under Linux on all platforms, though I suspect it has been well tested only on the i386 and perhaps the sparc Linux. It was developed originally on BSD systems based on the Mach microkernel, so it has been ported to FreeBSD and NetBSD; in the latter case again I suspect it hasn't been tested as thoroughly on some of the less common platforms.
Recently Solaris ports were announced, for the sparc and i386 platforms; these are fairly new, and not as well-used as the others.
Coda can be compiled in the CygWin environment from Cygnus, and the resulting client and server does work under Windows 95 and 98, though there are known to be some problems with this. The Coda server works under NT as well, though the Coda client does not.