Have you seen those around? A lot of websites have them, they're neat little square icons, ranging 100px to 300px square? They're a nice unobtrusive way of advertising a website or a product. I don't mind them at all and I have been wanting to add them to some of my websites for some time. You should be able to see an example on the problogger.net website, last time I checked I think there were 8 spots there.
I went on the hunt for these and I found that you can do them yourself by hand, or with the assistance of a plugin, you could of course use a random ad rotator and do them with that as well. But there is also a great plugin that allows you to create these icons, manage them and sell them all in one package. The plugin gives you all the management features, it even auto rotates the ads on each spot and, AND it also takes care of expiration and renewals and all that good stuff.
The plugin I'm using is for OIOpublisher, but I'm sure there are a few more out there that would do the trick. Along my searches I found that maybe WP-Ads would do the trick as well but I haven't tried them out yet. OIOPublisher's plugin lets you even add their own network ads, or other ads from your affiliates or google adsense or some other advertising content.
So that answers one more of the questions... check this one of my list.