The banner ad index page at weblogs.com shows all 500-odd microbanners for participating weblogs, including the number of views so far, the number of clickthroughs so far, and the clickthrough rate.

If you're at all interested in this sort of thing, it's fascinating to take a look at these banners and seeing what kinds of art, content, and animation, correspond to high (>1.5%) clickthrough rates and which correspnd to low (<0.2%) rates.

This is a great case study, the kind that's usually very hard to find in the real world. If anyone knows of any more, please let me know so I can share (and learn)!