An app that let's people perform Mechanical Turk tasks in their spare time. Your job will be to find tasks that are easy to do on a phone and streamline the process. Tasks like deciding if a picture is NSFW comes to mind.

The app would be free, but you'd charge some percentage of Turk wages.

With this app, people can finally earn spare change using a $200 phone and $30+/month data plan!

Some ideas that have been going through my head that I don't see myself ever getting around to working on (beyond the initial "jot them down in a notebook" stage):

1. An iPad interface for reMail. Not iPhone, but, definitely something that people would use and exposes you (the OP) to what a nice ObjC codebase looks like (personal formatting nitpicks aside). Might be able charge for this, if you wanted; reMail's source is now Apache 2, iirc.

2. Or maybe take the old Objective-C code from Tim Berners-Lee's original web browser and hack it up to the point where it works (don't know how big of a task this really is) and stick it into an iPad UI that looks how WorldWideWeb originally did. Might get rejected for having a non-WebKit-renderer. And not iPhone. But, I think that it'd be a fun project to hack on and something that you can show off.

3. If you use hg and bitbucket, an equivalent of iOctocat. Or maybe bzr and launchpad, whatever. Again, could probably charge for it, if you wanted to.

I've thought that a sailing app would be great.

Using the compass you can put your iphone on the deck and have an arrow point you to your next waypoint that you have plotted on Google maps beforehand. You can also record the route you've sailed and load it into a google maps mashup when you get back home. Or you could use the accelerometer to record how good or bad the weather has been on your trip (the more a boat rocks back and forth the worse the weather)

I haven't thought out exactly how it should be, but I'm pretty sure it would be a neat app. If you find it interesting my mail is in my profile.

There's no reason to publish it for free, it's probably something people will pay good money for :-)

How about a free crowdsourced talking tour guide app.

Basically, something that plots your location (GPS) and which direction you're facing (Magnetometer/Compass) and if there is anything interesting available on the server, play it.

Essentially make it so users can explore areas of interest (say a city) in their own way, instead of a predefined route.

Having a way for locals to upload their own audiotracks so you get the "inside scoop" on everything would be brilliant too.

you know how we have those calling cards and you have to call this long number and then enter in the international phone number you're dialing? anything that helps that process along would be quite cool. I'm using skype at the moment but it still can't beat the calling card price and carrying around the card or remembering long international numbers is a major hassle for their use.

An app which allows me to control the music played by my iPhone from my laptop.

http://news.ycombinator.com/item?id=1228560