Picasa integrates nicely with Google's online photo-hosting service, Picasa Web Albums, by allowing album syncing, uploading, and access control. Album syncing allows you to change a photo (by retouching, changing the caption, cropping, etc.) and have the changed photo appear in seconds on your web album.
Your iPhoto library can be viewed in Picasa in "read-only" form. If you want to make edits to photos in your iPhoto library, you'll have to let Picasa make a copy of the photo and then edit it. Picasa also lets you upload photos to Blogger, create videos of your photos, and create a collage.
It's nice to have a solid option aside from iPhoto to manage our photo collections locally and on the web. Picasa is a free 17MB download.