http://www.ms-links.com/ offers a 4 to 3 exchange ratio with nice stats including referral url data which almost no banner exchanges offer.

E-Bannerx - Offers a 3 to 2 exchange ratio and the ability to sell exchange credits to other members of the exchange. Supports button and skyscrapper formats too.

LinkBuddies - Offers a 2 to 1 exchange ratio and has more than 30,000 member sites. Gives 500 free credits for sign up and is for family friendly sites only. Supports banners, 88x31 butttons, and 90x60 buttons.

http://www.adendum.com/Skyscrapers.htm Banner exchange with 2 to 1 exchange ratio that gives 2000 free exchange credits for signing up and 500 for every month of membership. Lists some of the top banners run via their exchange on their site as well.

Linkexchange/Microsoft's B Central

This network is not recommended. Its a 2 to 1 exchange ratio that probably is the largest banner exchange on the web. To join you will have to sign up for Microsoft Passport among other things. I actually had to use google to find the sign up page for the exchange.

Bpath/Hyperbanner - offers a 2 to 1 exchange ratio and 500 credits upon sign up.

Also see: this sites general traffic exchange section for banner exchanges by topic and for exchanges for other sizes such as skyscrappers, popunders, and buttons.

*Check out
the BXmegalist site for a large list of hundreds of banner exchanges if you need even more choices. Note some of the exchanges probably no longer exist.