I just joined the 21st century by buying an iPhone 4 last weekend. Since then I've had great fun with photo apps.
1. Pano will help you stitch together photos and saves the result to your camera roll. This one is tricky to use at least in bright sunlight because you need to line up guides from the previous photo with what you're looking at for the next photo. Also, you have to pivot the iPhone, never mind yourself, around a fixed spot. Any exporting or sharing you'll need to do on your own.
2. The whole world has Instagram. I was sitting at the bar eating dinner while turning photos into Instagrams and the guy sitting 2 seats down from me was doing the exact same thing. But that doesn't make it any less fun. You can import from your camera roll and it's easy to send to Twitter, Facebook and Flickr as well as through e-mail. You can also save to the camera roll.
3. TiltShiftGen is very useful for narrowly defining fields of focus like those needed to create miniature scenes. You can import from your camera roll and easily export to Mail, Twitter, Facebook and SuperPopCam (but not Flickr) or save to the camera roll. Here's an example:
4. 100 Cameras in 1 by Stuck in Customs (Trey Ratcliff) has the most beautiful clean & musical interface. It's also got the most poetic memorable filter names like "Going back for ice, cream when no one is watching." It takes some doing for me to find a good fit between the filter and the image. But when I do it's nice.
You can share on Facebook, Twitter, Flickr, Smugmug and send to Dropbox and Instagram as well as e-mail.
I've got four more apps that I'm trying out. I'll let you know how it goes.