I did a domain hack using .us, but i would say you could also use an action word plus your product name. 37Signals does it for most of their apps, backpackit.co

I did a domain hack using .us, but i would say you could also use an action word plus your product name. 37Signals does it for most of their apps, backpackit.com, campfirenow.com also the vitamin web resource thinkvitamin.com. The trick is to create something memorable for people. I also try to make its easy for people to say over the phone to someone. Take slashdot.org, if the app is for normal people a name like that is not the greatest.