While there are plenty of places where you can "test your idea", I have recently came by the few of them:
- startupSQUARE http://www.startupsquare.com
- Reincarnation of LazyWeb http://twitter.com/lazyweb
- LoveMyIdea http://www.lovemyidea.com
If you know something else that also works, I'd love to hear about them.
P.S. Neither one of these worked for me. :)