Well, now you can put these squares on your own site, and sell them via GoSquared, as well. It’s the traditional take on selling ads hosted by someone else, but started the other way around, so to speak. There’s inventory already. Here’s why you should use it, with the words of the GoSquared team:
GoSquared is perhaps the easiest way to purchase advertising on any Site. Simply submit your Square, a 140×140 pixel image, and choose some keywords. GoSquared matches these keywords with similar Sites on the GoSquared Network, and suggests the most potentially effective ones.
Anyway, do check it out, the about page is more explanatory. Will you use it?