One Block Off the Grid, the solar group purchasing company, has developed an online estimate tool that you can use to see whether your roof is appropriate for a solar project as well as how much it might cost you.
Here's what it looks like. (You just have to type in your address and play around with where you'd like the panels on your roof.)
The organization currently has grassroots solar campaigns running in eight cities. Its goal is to help to create community solar-purchasing collectives that keep the costs of investing in solar technology lower for those involved. Here's the list of affiliated communities (my own community in Bergen County isn't online yet, drat!)