Good platform for free ad exchange / cross-promotion?

[alexc]

Has anyone successfully used an ad exchange or cross-promotion with other developers to promote your (free) apps? If you can find other apps with similar demographics, that are not direct competitors, then a direct ad exchange seems much more efficient than buying ads. Is there a good platform to do this?

I've done it using house ads in AdWhirl before, and it worked quite well, but the challenge is finding other interested developers. Is there some sort of large-scale, automated ad exchange program anywhere?

Also, if anyone's interested in trading ads in blocks of 25-50k impressions/day, for a few weeks, send me a PM (anything less than that isn't really going to be worth the trouble to set up). My main demographic is female, 14-24, 85% US traffic.