Java runtime environment.
Sun Microsystems has one,
Microsoft has one, and of course there's
Kaffe, the free Linux JRE. In addition,
Netscape and
IE tend to tweak out their JRE's in slightly different ways, meaning that compile-once-run-anywhere isn't quite all it's cracked up to be...
Oh well, at least there's still POSIX for cross-platform devel.