I didn't apply, but because I thought my business wasn't mature enough for YCombinator. BUT last time I started this thread, I was criticized for not being as forthright. So! my bootstrapped startup:
www.CameraLends.com, P2P camera rental. We've reached 500 users and nearly 100 transactions to date (and growing and accelerating week-over-week). We did not apply to YC because all YC alums who gave me advice said that YC is looking for "10% week-over-week growth" and our numbers are low enough to not be able to show that consistently.