Quid Pro Quo podcast advertising

When I read this post by Jay it got me thinking. He posted an audio ad for his own podcast and asked anyone to include it in their podcast and that he'd reciprocate by playing their ad in his podcast.

Which made me Google around a bit and now I'm asking: is there a site for ad-trading like that? Seems like it'd be a nice thing to have especially for the more hobby-level podcasts. You could organize straight-up paid advertising but then also simple ad trades between podcasts.