The Best:

LinkExchange

 The original banner exchange network. LinkExchange will display your banner once on its network of sites for every two banners displayed at your site. We have been using LinkExchange for our site and we are quite pleased with the service.

TrafficX

 Unlike other banner networks, TrafficX trades click throughs among its members. You run TrafficX banners on your site and for every 2 click throughs generated from your site, you earn 1 click through to your site. So, instead of just generating impressions, the TrafficX system generates visitors to your site.

The Others:

123 Banners

 For every two banners displayed on your site, you earn 1 display of your banner on the 123 Banners network.

BannerSwap

 For every two banners displayed on your site, you earn 1 display of your banner on the BannerSwap network.

BannerWeb

 For every two banners displayed on your site, you earn 1 display of your banner on the BannerWeb network. With BannerWeb you can simultaneously have up to seven different banner ads for your site online. BannerWeb's intelligent ad rotation system will track the success (click-thru ratio) of each ad and continuously adjust the rotation to display your most successful ads the most often.

Free-Banners

 For every two banners displayed on your site, you earn 1 display of your banner on the Free Banners network.

SmartClicks

 For every two banners displayed on your site, you earn 1 display of your banner on the SmartClicks network.

Soul Exchange

 Banner exchange network for African American oriented sites. For every two banners displayed on your site, you earn 1 display of your banner.