Want to emulate that neat collage look you get when you get photos printed out? There's a service called Collagr that does it for you pulling in photos from Flickr. If you've got shots on there, just fill in your username and Collagr will do its magic. The result is a square array that may look a little small in the preview, but if you save it to your hard drive you'll notice it's 1,400x1,025, which is larger than the average desktop wallpaper. The recommended use, however, is for MySpace profiles--you know, the kind that are nearly impossible to read.
Collagr is working on adding other photo-hosting services, and hopefully a way to grab entire sets, too. Ideally, I'd like to see these guys come together with custom sizes, and a drag-and-drop interface that lets you put together your own contact sheets.
Here's an example I got from one of my favorite Flickr users, Ulterior Epicure--also known as the guy who posts nothing but pictures of gourmet food.