Think about what your application's supposed to do - what are its benefits for users, then come up with some metaphors for that. Should be terms that give people a general concept of what it is that you're offering. Then use the various domain search engines mentioned here until you find a free domain. BTW, make sure you can get the .com, .net, .org, and that all the rest (.biz, .us, .eu) are free. Once you're certain you have a name you'll stay with, register a trademark ASAP.