Ask HN: ideas for an iphone app that uses a web service?

I had planned on building a mobile service for a startup I had stuck in my head for a while, but I've given up that idea because of complications I came across while thinking it through.

I still want a project to work on while I think up a problem I want to solve, so I'm taking ideas!

I want to learn to build a web service and and integrate with an iphone app, similar to Instagram or something like that, and I have some great tutorials to learn from, I just need a project. I would like something that will end up being useful and I can actually upload to the app store, but I'm open to project ideas that are simply for "shits and giggles" as well.

Thanks,

Alex G