Java Serial Communication on Windows
Solution 1:
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.
Solution 2:
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.
Solution 3:
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.