A ton of blogs have details on the incipient announcement of a Yahoo contextual ad program for content sites (i.e. AdSense from Yahoo).
Here's what I'm gonna do as soon as I can sign up: Put all Yahoo ads up on even days and all Google on odd days. In other words alternate the ads in some way that complies with the TOS. I believe Google's TOS is exclusive - you can't put other context ads up on the same page.
Why? Because the Google ads are too repititive on this blog. You see the same ads all of the time. I'm too lazy (so far) to explore the deep options on the Google interface to try and get different ads. But if I switch systems from day-to-day, then maybe I can reduce ad-fatigue... Who knows? But it should be fun to try...