Good resource for ideas. Personally I've tried some things that fall into the list but nothing has taken of yet. Some of them didn't make it beyond proof of concept.

The one I spent the most time on was a few years ago for airline flight searching. The main idea was to allow people to track the price of flights they were planning to take to help them pick the right time to buy. I think that falls in "20. Shopping guides". I spent a long time finding a flight search API that was available to a small timer like myself. I eventually came across the Kayak Search API that was perfect for my needs. I got as far as a working proof of concept when I saw this post. Although farecast is not quite what I had in mind that was quite a blow. The final nail in the coffin was when Kayak started showing flight price graphs. Now I see recently released yapta.com which is pretty much exactly what I had in mind. Maybe I should have kept going?

Still that experience made it easier for me to join the facebook app gold rush with another airline flight idea Fly to Friends & Places. It's still going with about 14,000 installs. Not bad but pretty small fry by facebook standards.

So... onto the next idea. Probably a good idea to try something non-flight related.