After using RXTX for a number of different projects, I\'ve come across many annoying discrepancies and issues that can only sensibly be put down to bugs in the library - dea
There's a few of them out there now, now that Arduino is pretty popular.
There's Serialio
and PureJavaComm, but it's pretty early on in development from what I can see.
it really depends on what your needs are. RXTX has been ported pretty much everywhere, so it is widely available.