With the recent Text Link Ads and Google slap-related panics, I was wondering if there should be (or if there already exists) a site where groups of people in specific niches can form together and collectively advertise their sites, giving advertisers a place to go to find a range of sites in a field. The site should just be a free introduction service, no middleman fees, so people could have an extra outlet to advertise space on their blog. Does such a think already exist?