Before you let it rest in your project folder, you can try to pitch your idea at a startup weekend or similar event (basically, hackers, designers and business people spend a weekend trying to "create" a startup, usually building a MVP and doing some customer validation).

If your pitch includes "I have a mailing list of X persons who are very interested in this and would love to spend money on it", then it is going to stand out. It is usually possible to build a really rough proof-of-concept in that time, and you can use that to validate your idea, to a certain extent. If you are lucky, you might even find some developers who find this project interesting enough to continue working on it after the weekend.

But you'll need some luck for this to work perfectly, so if you are super-serious about it, try the paths outlined by others here.