Startups usually start either with a solution, and then seek paying customers, or they start with a clear market problem and build a solution for it. Both a viable strategies, but I much prefer the second approach.
I recently learned about a neat startup built around this second approach: Onavo. Bandwidth caps for mobile phone data usage are already here for many users, and are coming for the rest of us. Onavo compresses your web traffic so you can get more bang for your bandwidth buck.
GigaOhm did a great job describing the company and its space here: Honey, I Shrunk the Mobile Data Use!
Hat tip to Jay Leonard.