Idea for a useful webapp: A tool for doing web page mockups that's better than Photoshop because things actually look right (because it's rendered by the browser). It doesn't need to generate good html, so absolute positioning, etc is ok.
I'd like to reiterate my call for Facebook to open source the Friendfeed codebase (as AOL did when it bought Netscape). A federated OpenFriendfeed is the best for users - and fits Facebook's move toward an open social platform. I'll pay to run a server. Would you?