I do a lot of photo editing on my iPad. A LOT. My camera has an Eye-Fi card in it that auto-uploads every picture I snap to Photobucket. I use the Photobucket app to sort through those, put them in albums, etc. Then I download the ones that need fixing up and do that on the iPad and reupload them with the app.
I have literally a couple dozen photo editing apps on my iPad, but over time I've found that there are just a few that I keep going back to, the workhorses that are good for more than just a few gimmicks.
Snapseed - There's a reason this app won app of the year last year. We went to Hawaii this year, and the photo problem was that there was tons of nice scenery, but it was overcast and rainy almost the whole time so it was hard to get a nice shot of it. Enter Snapseed, with their Drama filters and excellent tuning tools. Over and over again, I put in a photo that was too dark, too underexposed, and it found whatever information was hiding in there and brought it out to great effect.
Photo as originally taken:
After running it through Snapseed:
Snapseed brought out details of the island that I thought had been lost, and made the water look more like it had before the clouds rolled in and dulled its surface. And it took just a couple of minutes!
PhotoGene - This is actually a recent purchase, but I'm already using it constantly. I bought it for its redeye correction - the Photos app can catch some red eyes, but sometimes it overdoes it and you wind up with eyes that look like they came out of a horror movie. Rather than simply putting black blobs over the red, PhotoGene adjusts the color to a yellower hue and darkens it. And if that doesn't work, it has a dodge mask feature that lets you darken just the bit you want, to get rid of remaining pupil glare. And if that doesn't work, it has a cloning heal tool, and I will freely admit that I just printed out a couple pictures that had one iris cloned over to the other side to cover up an irretrievably red eye. You have to be careful with that, of course, or you'll wind up with some very creepy results, but if you're careful you can't tell, especially at 4x6 size. The masking tools are useful for a variety of other applications as well, and I have barely begun to explore this app's other features.
Here is an example of my dog Fred before:
And after redeye correction and a burn mask in PhotoGene:
As you can see, the Photo app correction is a little too harsh in this picture; I find it works best when the pupil is well-defined and is the only thing that's red:
PicStitch - I assumed that a collage-making app would be a gimmick I'd use once or twice and forget about, but this turns out to be a very useful tool. When I have a variety of similar shots that I want to share online (say, of my baby trying different foods and making a mess out of all of them) I don't have to make my FaceBook friends flip through a whole album. I even print them out sometimes - I wanted to capture the interesting local options on the Waikiki McDonald's menu, but didn't really want to print out four photos of a McDonald's, so instead I printed this one picture:
