I built http://www.sparkmuse.com just for that purpose. We've gotten nearly 100 ideas now, but not alot of traction, so I wouldn't build another :)

You don't have a lot of traction because you don't show how you add value. If you need a signup before getting to the app, you're already behind the 8 ball. The best you can do is to make the landing page really sell me on why I should do that.

I saw your website in the past but I wish I could see screenshots of what you intend to provide with your service before asking me to sign up.

There might be something there, thanks for the suggestion.

I totally agree, could be like formspring.