http://free.LocalDataPlace.com - which we built for local recycling (ala freecycle). We haven't been willing to spam existing freecycle groups but we need good local traction to catch on.
You should look for groups focused on recycling and sustainability and promote your site to them as a way to do that. Obviously don't be spammy, but if you get involved in the overall discussion then people will be interested in what you have to say about your site. Look around Google Groups for related communities.
If it's having trouble getting nationwide traction, just focus on a specific region initially (such as where you live) -- do some networking and get the site name out there.
I've got something similar to that - http://relovable.com/ - designed to target the parenting niche. Kids grow (zing!) through clothes at an alarming rate.
I never bothered to attempt to market it at all - it was more a proof-of-concept that I could do something with Rails. Still think it has value though.
