This might sound overly simplistic, but...scratch an itch.
Is there anything you want that's not out there, or not out there to your satisfaction? For example, I'm not happy with bookmark management in Firefox. One day, when I have downtime, I'll right a small web app that lets you upload a Firefox bookmark file and then creates a site with everything properly linked and formatted. It will also check for dead/redirected links, etc.
Almost a mini-delicious/squidoo
For all I know such a script exists, but I've never heard of one. And even if it does, I want to write my own, one that does things my way.
So...what's your itch?