It might sound too obvious but often people try to come up with "ideas" instead of building what people need/want. I am pretty sure you just meant to ask: what should I build?

To find out what people need/want there are different ways to find out: Sites like: http://www.43things.com/zeitgeist/goals might help.

Also take a look at PGs "Ideas for Startups": http://www.paulgraham.com/ideas.html