Windows Vista SP1 (Service Pack 1) is the next update to microsoft's Windows Vista Operating System. This update "addresses reliability and performance issues, and provides support for new hardware and several emerging standards" (Microsoft)
The update is currently only available to Technet and MSDN Subscribers, but it will be available as a free download from windows update in "mid-March".
I used the beta of the service pack in December and I thought it was well worth it. Of course, this time around I wont have a branding on my wallpaper.