There are a lot of pages that provide for free many startup ideas (a classic example is http://ycombinator.com/ideas.html but you'll easily find several other pages full of ideas). Maybe you can get some inspiration there?
In the end, the idea is only the first step in your business and arguably not the most important one.
1) solve a problem for yourself... "need" is the mother of invention.
2) copy another idea... if you are good at executing. (Jordi's link)
3) immerse yourself in a specific sector of interest... "the best thing you could ever do for a job is match your skills, with your interests, with a market opportunity" --Tina Seelig, Director of Stanford's Entrepreneurial Thought Leaders Program.