David from 37signals posted a blog entry about people who have succeeded with ideas you may have thought of a while ago.
He's right about it all of course - an idea by itself it worth nothing. If you do nothing with it, then you have nobody to blame but yourself if someone else makes a whole lot of money from a similar idea.
Of course this doesn't necessarily stop the nagging feeling that it could have been you with all that income...
My big idea that I did nothing with? I think it was around 1998 when I thought it would be useful to have a real estate website to which rental agencies could submit available properties complete with photos and searchable descriptions. Seriously. Oh well.