1) Shared Places
The idea is that you can have public or private lists of locations that can be shared with other people, and would be integrated with mapping software on your phone so it is easy to navigate to them.
Some private lists that may be useful:
a) A family one that keeps track of extended relatives, friends houses, soccer fields, etc
b) A small business that works at, delivers to, or services fixed locations.
Some public ones might be:
a) Places in this area that are related to US Civil War history.
b) Gas stations in this area that have a drive-thru (this sounds silly, but was actually where I got the idea - my sister has small children and it is a pain to get them out of the car and into a store to just pick up a few convenience items, so knowing where she can go that has a drive-thru is really handy)
2) Insomnia Tracker
An app/website to log things related to sleep (beverages consumed, medication taken, notable social situations, what time you try to go to sleep, what time you wake up, etc). Hopefully having it available on a phone will make it easier to keep track of things more fully than if you had to remember to write it down in a paper log, since you usually have your phone with you.
Then this data can be used to find trends such as how, say your coffee consumption affects your sleep, or maybe how stressful arguments at work are keeping you up at night.
I don't have anything up for either of them yet, and I'm not sure either could actually make enough to be more than a hobby, but they are both useful to me and kind of fun to build.