It's probably the most interesting feature of Picasa Web Albums: you can monitor all the photos uploaded to your contacts' albums.
If you use Google's photo sharing site, you can view pictures from the most recently modified albums and from your contacts. You select a list of folders that contain photos and Google Screensaver monitors the JPEGs from these folders. Google Screensaver is a Windows screensaver that displays photos from different locations:
Update: The screensaver is now a Picasa feature.
