The folks over at mozilla have a really cool Javascript sniffer that enables you to detect the browser's vendor, version number, and operating system. It has been tested on Navigator

2, 3, 4, and 5 (developer preview), Internet Explorer 3, 4, and 5, and Opera 3 on Windows 95, Windows NT, the Macintosh, and

SunOS5. It is believed to be compatible with all JavaScript-capable browser versions on all platforms.