OpenSource Banner Exchange?

TimRiker asks: "I run the Open Source project BZFlag and would like to increase awareness of its existence. I see ads for Open Source projects on Slashdot, SourceForge, Freshmeat, etc. Is there an OpenSource banner exchange around that will get my ad shown on other sites in exchange for displaying their ads on my site? I'd prefer not to show adds on my site for any non-free software." This is an interesting thought and it would go about "getting the word out" in an efficient manner. I haven't heard of such a service, but I think it might be a good idea. What about you?