I took a look at quite a few of the sites recommended by the 26.2 ers. I like the idea of a trading web site, to let you trade used books or CDs, although the one-by-one transaction approach isn't going to work with web sites that keep lists of what is available once it gets a lot of traffic, you really need searchable databases or the thing won't scale.
I went to Five Limes with high hopes but the Boston resources were slim to none. I could think of lots of things to do at a site like that so I'm glad it was mentioned (find green contractors, figure out what the cost/benefit is likely to be for switching from oil to gas heat, check a carpool network, trade tips about the best composting systems) but the site seems still somewhat underdeveloped for the sorts of things I would be the most interested in. I wish it well - I hope it takes off and plan to check back!