Another great one is Mandolux. Almost all of his pictures are in a variety of formats, and is especially useful if you have multiple monitors. And, despite being very random most of the time, they're usually quite interesting.
Thanks for posting this, guys, I've had a box.net account for a while but no easy way to access it. I tried that German one, but it was kind of slow and took a bit of guesswork to register (given that the entire site is in another language), and is much faster (though still not fast in general.) Here's my referral link, link. Thanks muchly.