Note: this post initially linked to a Greasemonkey script that no longer works, but it now offers an easier solution suggested in the comments.
Unlike other photo sharing sites, Picasa Web Albums offers a way to download an entire album, but that requires Picasa 2.5 or later. If you don't want to install the application or you use Mac or Linux, there are other ways to download a photo album.
For Firefox, you can use DownThemAll, probably the best download manager add-on. After installing the extension and restarting the browser, go to the album you want to download, click on the "RSS" link from the sidebar and you'll get the album's feed. Right-click on the page, select DownThemAll/DownThemAll in the contextual menu, expand the Filters section and check "Images". Don't forget to enter the folder where you want to save the photos.
Update: Read the comments to find a lot of interesting ways to download albums from Picasa Web.