Some in the current cycle are instances of them. For example, AirBnb is #25, the Craiglist competitor (a suggestion which iirc was much ridiculed at the time).
I think jacktang might also want to know that you still want to fund startups with those ideas -- in which case I think jacktang probably wants to see this: http://ycombinator.com/s2009.html
YC, I am a bit confused with the sentences:
"$5000 + $5000n is not a lot, but it turns out to be enough. It will cover at least 4 months' living expenses, and that is enough time to build something nontrivial."
After 4 months, what milestone should the team arrives so that you judge the project is promising? should I prove the business model is correct in the 4 months?