Here's a list of companies to get you started. Just click on the company's name to go to a page with a description of the company and a link to its web site.
This is a company based out of MI that has an interesting business model. They utilize a verticle axis technology that can easily fit on an urban roof top. I spoke to one of their customers who told me:McKenzie bay owned,installed, and maintianed the whole system; the customer had no upfront costs; the customer signed a 10-15 year contract to buy back the power from the company at a pre-arranged prices which were quite attractive when compared to the local utility charges. After the contract period the equipment would be owned by the customer and they would be responsible for maintaining it.
The interesting part of this is the business model as well as the fact that they have developed an entire solution. As a customer you have the option to just buy power from them generated on your site or you can buy individual components.