GLX-Dock 3.1 is the new release and it brings in better integration of Unity, all configuration windows have been merged into a single window, progress bars in several applets, the music applet can control players from the system tray, icon separation in the taskbar, improvements to the advanced configuration panel mode, a rewritten messaging menu, and various bug-fixes / enhancements.
The better Unity integration brings support for the Unity launcher API and better support of Unity indicators. New GLX-Dock indicator applets include the printers menu showing active print jobs and the sync menu showing information from processes that involve different synchronization forms.
Some other changes include the dock now supporting RandR for better multi-monitor support outside of Xinerama, plug-in improvements, the Twitter applet supporting Identi.ca, and much more.
More details on GLX-Dock 3.1 are available from the GLX-Dock.org forum announcement.