Re: Photo apps
My everyday, go-to editing app. Absolutely amazing in both how much it does (color correction, crops, sharpening, center-focus, tilt-shift, filters, etc) and in how well it does it.
More complicated than Snapseed, I use these not for everyday edits, but for when I need something like Layer and Masks support (Photoforge2) or a Clone Stamp (Photogene2). I also use Photoforge2 for its Curves support--great for tweaking color images with fine control. Probably more complicated than most users want.
The only app you'll need for text-on-photos. Add as much as you'd like, import your own fonts (a must for anyone with any interest in typography--no app will have it all), style your text, adjust kerning and line spacing, opacity, and more.
Create photo sets of multiple images, using over 50 different adjustable layouts. Change border color and width, corner radius, etc. Great for side-by-side photo comparisons as well as more artistic collections.
(screenshots made with PreviewMaker; side-by-side with Diptic)
Superimpose: Similar to Photoforge2's Layers/Masks options but easier to use in many situations. Most often I use it to isolate an image, delete the surrounding background, and place the remaining image on a new background.
(Superimpose used to place R2D2 onto a Disney drawing; which was then aged with Snapseed to help blend the two.)
Create black-and-white line drawings from your photos (color can be painted back in later if desired). Great controls for adjusting line width, black/white/grey ratios, etc. Much more refined than most "cartoon-ify" apps.
(ToonPaint used to turn a photo of my dog into a drawing; Photoforge2 used to insert him into the album cover.)
PreviewMaker/iWrapper: A bit of a specialty app, but one I use often when reviewing apps on my site. Both of these use your own screenshots and place them in an image frame for a device of your choosing. I began with PreviewMaker when it was a Cydia-only app; Apple has recently reversed its stance on using iOS device images in screenshot apps so it's now available in the App Store. However, it hasn't been updated for the iPhone 5 yet--so for now I'm using iWrapper. Very similar, but I prefer the workflow of PreviewMaker.
I just discovered Glaze, which applies painterly effects to photos. The big plus here is that it does a much better job than most apps. Downside is that the controls aren't very adjustable once you're in a given effect. (You can, however, layer effects.) Here is the shot from above, passed through a Glaze filter.
(a single pass through one of Glaze's effects.)
ToonCamera does a bit of what ToonPaint does, just not with the same attention to detail. It does, however, give you the option to use it in video mode, which is a very cool feature. It also provides a live preview, so you can see what your sketched images will look like before taking them.
Imgupr is a great way to upload images to Imgur. Easy to navigate and quick, it also makes it simple to copy the IMG codes used in forums like this one. Worth a look if you do a lot of screenshot attachments in places like iMore and don't have a good system yet for posting from your mobile.
GifBoom is a handy way to make animated gifs. It comes with a built-in social aspect that I never use as it's overrun with junior-high kids, but the gif creation works well, letting you use video, images, or text to make your gif. It also provides you with Instagram-style filters, as well as backgrounds, borders, fonts, etc, and can export to the usual facebook/tumblr/sms and so on. There are plenty of other gif apps out there (Cinemagram is a cool variation that allows you to animate a single part of a video, leaving the rest as a still image) if this one doesn't fit the bill.