The great promise of any programming language with a virtual machine, most recently Java.
Actually, and this is the voice of bitter experience talking, kiddies, it's Write Once, Port Everywhere. Write on Windows and port to Linux or the reverse. Then port to Solaris. Then AIX and HP-UX and so on ... Argh!
James Gosling recently said: "The perfect goal of "write once, run anywhere, anything runs on
anything" is just goofy. You're never going to run some piece of weather modeling software on a toaster (laughs). And you wouldn't want to. So there are some scale and capability limits. But within that, you can do
an awful lot to make sure that if somebody wants to read a file, it looks the same everywhere reading a file makes sense."
For more see http://www-4.ibm.com/software/developer/features/gosling/index.html