[amorphid]

The JVM is available on Linux and Mac OS X, too.

[mtogo]

Sure it's available, but it's error-prone to install, often an old version, and generally breaks all over the place.

[Aloha]

There are other implementations too, like IBM J9 and GNU Kaffe. I remember using J9 in place of Sun Java when I was a desktop Linux user.

[chc]

Oracle has demonstrated a certain eagerness to sue alternative implementations, so I'm not sure those really get around the issue