JBother is a Jabber client written in pure Java (1.4.x). It supports groupchat in a tabbed window, multiple resources, transport registration (so you can get on AIM, MSN, Yahoo!, and ICQ through JBother), sounds for different events, has several different skins, and should run on any platform that has the JRE installed (I have personally seen it working on Windows, Linux, FreeBSD, Solaris, and OS X).
It has a plugin system that allows us to write code that is specific to a certain platform, while keeping the core pure Java. An example is the Win32 plugin - it allows for an icon in the system tray on Windows. All plugins can be easily viewed and installed from within JBother's plugin manager.