I have been using purejavacomm:
It is an implementation of javax.comm written in pure java + JNA
Unlike rxtx, you don't need to install a dll. It is written in pure Java + JNA, which solved the problem of portability between Windows and Linux for me. It should be easy to port to other OS-es that JNA supports, such as Solaris and FreeBSD, but I haven't tried it.
You might expect a pure java library to lag behind a native implementation such as rxtx in performance, but with modern CPU's, the bottleneck is very likely to be the bitrate of your serial port, not CPU cycles. Also, it's much easier to debug than a mixed Java/Native library or pure compiled native code.
