VSCO is my favorite app for processing photos on the go. My favorite tools to use on VSCO are the exposure tool -- to fix the lightness and darkness of images, and the temperature tool -- to make images either cooler or warmer. The filters all all really good, too. I don't have a favorite as it depends entirely on the mood of the photo that I'm processing.
A VSCO feature that I only recently discovered has my mind blown as well--if you take a photo with the VSCO app, you can use two fingers on the screen, one to adjust exposure and the other to adjust focus. This capability gives you almost DSLR-like capabilities right from your smart-phone. Pretty great.
2. STUDIO DESIGN
The ability to add text to photos right from the smartphone is extremely convenient--and STUDIO does a great job of providing nice fonts, graphics and an easy interface so that even novice graphic designers can give their images a nice graphic touch to compliment great photos.
The Steller app is way to create a mini digital magazine within a matter of minutes. If you've got a story to tell and you want to tell it with words, video and text, Steller is is the perfect platform.
Pippit is a brand new app that is like an RSS feed meets Instagram--and you can add 'pips' to posts to include rich info about items for your readers. A great tool for bloggers who are looking to capture their mobile audience.
5. EOS REMOTE
If you have a canon EOS with wi-fi capability, (I know most of you probably don't but this is so cool I had to share..) then get the EOS Remote app and you can shoot from your phone and view photos being taken on the camera in real time. This was an amazing tool to have while we were shooting the images for my new interiors book because I could art direct without being tethered to a computer or without hovering over the photographer (Dabito's) shoulder. It's kind of amazing.