There are a couple of other services which are more "client-suitable" and have a few more options:
* http://dummyimage.com/ * http://placehold.it/
The most useful thing to come out of this post for me is the knowledge of dummyimages.com, and specifically the aliases for the IAB standard ad sizes such as http://dummyimage.com/leaderboard/E/C. Great work by Russell Heimlich on that site, and the source code is MIT too if you want to run it locally.