there were several times where i'd get really excited about one idea -- like pacing in my living room at 5:30am excited -- and then 5 days later find out (via a different set of search terms or something) there were 3 other people doing the same thing, with a head start and more money.
ultimately they say scratch your own itch -- this was a problem (syncing a 3gb file across several computers efficiently) i routinely had working on a prior company i had started and i was frustrated that no one had solved it well, and it turned out to be more promising than my original company :)