for me project ideas come from either problems I've experienced myself that I want to solve, or some app that I know I want.
In the case of http://tweetsaver.com, there was no other solution at the time it launched (over a year ago) that offered a decent way to back up my tweets, as well as search all my old tweets. I knew that, had there been another service at the time that offered this, I would have paid to use it.
Most of the ideas that I have that have come from speculating on what might be theoretically useful or super successful/popular are still just domain names :-)