There's also the lovely and whimsical http://placekitten.com/

Also http://placedog.com/ and for the refined taste, http://placebear.com/ .

This is my favorite by far.

Interestingly that page never stops loading for me and drives Chrome nuts. It keeps loading tons of different images and the icon for stop/refresh alternates between the two states a couple times per second.

I saw that too. When looking at the Chrome developer tools you can see a bunch of stuff being loaded.

Something weird is going on with the favicon for me (Chrome 25.0.1323.1 dev-m). It's kind of flashing/glitching. Anyone else?

Don't forget the infamous CageMe: http://cageme.herokuapp.com/

I didn’t know that one, haha.