YC Rejected: Please read.
"Why did I get rejected?!??"
I don't know, many of us don't either. Other than your idea possibly being a pile of garbage, none of us here know as to the exact reasons you got rejected. Sure, pg will, but I doubt he'll write up a critique of your app and post it publicly. If you want to know why you were rejected, email someone at yc to see if they're even willing to give you feedback.
"Maybe we should start a YC-rejected group!!!"
Go for it. Honestly, it's a big waste of time. Last year someone created RejectedByYC.com, but that seems to have vanished. I don't really know why you'd want to limit yourself to giving/receiving feedback and support for your project from other YC-rejected companies. The rest of us who got accepted or didn't apply aren't less willing to encourage/support/critique your efforts.
"I got rejected, but I'm moving on!!!"
Awesome, but really, even considering to discontinue a project based on your YC decision is foolish. Sure, the experience is great, you get a little cash to get you off the ground, but if your motivation was that dependent on the outcome of whether you'd be accepted to YC, you need to realize that there are much higher hurdles to be conquered ahead. Plus, we don't need to know you're moving on. Create something, ask for feedback, and we'll care.
"MyStartup (YC reject '09) asking for feedback!!!"
Don't label yourself as a YC reject. I don't know why people do this. I am no more interested in your startup whether accepted or rejected from YC. There are several YC companies I think are crap, and many rejects that I think are awesome. I guess what you're trying to get across by labeling yourself as a YC rejects is "PG thinks we're crap, but I'm out to prove him wrong!" That makes you seem bitter and unable to emotionally settle with your rejection. We are a community of startups and people interested in them and we need not to label ourselves into various camps. There are startups that are accepted into YC, and for those companies, they were each merely granted an opportunity. For every YC accepted company, there's another who has received angel investing sans incubator-type program, and another who has been hooked up with a client that could bring lots of business, and another that just took vc funding. Starting companies is about finding opportunities. And if YC isn't an opportunity that came your way, you have hundreds of other paths to pursue.
I've said this before, but please realize that Digg, Facebook, Twitter, Google, YouTube, Myspace, eBay, Amazon, PayPal, and 99.999% of the most popular and/or highly valued acquired companies didn't go through YC, nor any other YC-like program.