this blog post about it.
The Biggest Sites of That Kindhttp://AngelList.com/ - a big site, suitable for bigger projects, the minimum is $150 thousands of capital.
http://selfstarter.us/ - here you can download an open source crowdfunding platform, install it on your server and make your own site of that kind.
http://kickstarter.com/ - a great and very popular site, where your idea has a good chance to be noticed.
http://indiegogo.com/ - another big site, where you will receive the money for your project, even if you don't reach your money goal. Additional information on the matter you can see on Wikipedia here.