Java Serial Communication on Windows
I started looking for the same thing couple weeks ago, and I've been very happy with the multi-platform RXTX library so far. Works with any Windows, Linux and OS X. Has a very clean, easy to understand API.
edit: RXTX is also open source.
Without reservation, I recommend Java Serial Port from serialio.com; I had significant stability problems with the Sun, IBM and RxTx serial package. SerialPort has been rock solid in production 24/7 for over 5 years.
They support the standard Java serial API, as well as their own alternative proprietary one. I would stick with the standard API though, unless you really need something theirs has that the standard one doesn't, just to keep your options open.
I've been using PureJavaComm for the last five years or so. It's actively maintained, and is a pure Java (via JNA) implementation. RXTX has let me down because of a lack of maintenance and numerous subtle problems.