I've been in the middle of working on a start-up for the past 8 months or so, and during that time I've only had one idea that I thought to myself "That idea is worth a lot more money than the one you are working on right now, and it would be easier to engineer."
I wonder if this app already exists. If it doesn't, feel free to run with it if you see the same potential I did.
The app is this - a button to turn a Flickr account into an online store, where people can order prints or pay for the rights to use photos. Everything starts off at a dollar, but it's fully customizable.
Having tinkered with the Flickr API, I don't think this would be hard to engineer. And I think there is an easy revenue model (a cut of sales), a low barrier to entry for users, and a large long tail to make money off of.
So anyways, that's it. I know ideas are worthless, but if you want this one, it's yours.
EDIT: I'm working o n a website for hikers - www.trailbehind.com. Probably less money in it, but it's something I'm passionate about.
Sold to Cafe Press recently ($20m I believe).