After finding a bunch of exchanges that give you only 5 impressions for every 10 you display, I found one that gives me 8. They even set up a sports category for me for my cavsworld site. I'm showing a better click through rate for that than I was for my ad words campaign and it's free. Check it out

http://www.yearpixels.com/bannerexchange/

I put it at the top of my cavsworld site

http://www.cavsworld.com

and the bottom of my cafeshopper site

http://www.cafeshopper.com

They let you decide if you want to show all banners or only ones in your category. I went the all banner route.