Matt Cutts on his blog talked about how to find start-up ideas. It is pretty interesting stuff.
He suggested that you walk around your house and look for hot spots. A hotspot is where high information density can be found. Like your music CDs (iTunes, Amazon MP3 store, doubleTwist, MP3tunes, etc.), bank statements (Mint), photo albums (ScanCafe). Each hotspot can be turned into start-up ideas.