A number of improvements to the client-side Office applications as well as a few to the server:
- Variations: Performance and manageability improvements including STSADM commands for repairing links between source and target pages
- Content databases: Improvements to read-only content databases and index rebuild timer jobs in Windows SharePoint Services 3.0
- Search: Just some great love for improving performance, reliability, and accuracy
- Browser Support: Microsoft acknowledges that Firefox is used to access SharePoint and shows them some love as well.
Service Pack 2 can be downloaded here: http://www.microsoft.com/download/en/details.aspx?displaylang=en&id=16679