Manage your photos in one place, and find photos you forgot you had
Turn photos into movies, collages, slideshows and more
Upload seamlessly to Picasa Web Albums to share with friends, family & the world
Supported on Windows XP, Vista, Mac OS X, and Linux. Making it genuinely cross-platform.
Features may vary depending on your platform.
On the Mac version it will allow you to import your photos from iPhoto to: upload, create a collage, create a movie, email, and print, (in beta) it's set to read only mode in order to prevent it from messing up the iPhoto library structure, but that just means that you can't edit, move, delete, add tags or use the sync to web feature, for the pictures that are stored only in your iPhoto library on your Mac.
On Linux it can be downloaded and installed through your favorite package manager using the google repository, as depicted in this article of the Picasa Linux FAQ
Windows, has a feature for creating and burning slide shows to CD.
As with most things from Google beta is indefinite.