Yesterday, Mac OS X 10.5.6 was released, and the 190mb update includes:
- Stability and Performance improvements to OS X apps (Mail, iCal, Safari, Airport)
- New Trackpad System Preferences panel (only applies to laptop Macs)
- DVD Player performance improvements
- Graphical Quality improvements
- Time Machine improvements
- Security Updates
The support page details all of the updates, improvements, and tweaks.