This could be a good start up idea?
For example, a search engine for chef's looking for new food to cook. A search engine for auto-repair for do-it-yourselfers. And of course a programming one. Basically, any major topic or profession.
This could be done by taking a large engine (such as Google) and customizing its queries, filtering out non-relevant stuff. Because currently everything is bunched up together in one giant index. It's not as efficient as it could be.