There is an article on Pay Per Click Analyst discussing some different verticals to try in a PPC campaign.

With the bigger engines starting to get into a big fight against each other, the smaller guys are trying to reach niche marketplaces. Two smaller programs discussed in the article are Quigo and Industry Brains. While advertisers won't get nearly the same volume they would with Google or Yahoo!, certain search campaigns can get a small boost from using these PPC programs (name a company who will turn down extra profits).