crunchmob.com <-- considering buying this one
I've considered having a best available .com competition here, but think it might be considered wasteful, doing the work of squatters for them.
I've grabbed it via 1&1 - if you're going to do something useful with either of them - let me know so I can transfer it to you gratis - just create something interesting. :-)
...I like the ykombinator premise too.