If you're like me and end up with free time on a train or somewhere away from the computer that could be better spent working on your project, then you may find these programs useful:
Pixly (Android) Free (donate to remove ads)
Pixly is an amazing little powerhouse app for creating sprites, tiles, backgrounds, or anything pixel art on android. It supports layers, animation, custom palettes and has a set of great tools. The developer is a cool guy who is usually quick to respond to any concerns or suggestions. You can also export scaled images and gifs as well. I've been using this for a while and so far it's been one of the best pixel art programs I've used and the best on android.
A cool thing about Pixly is that even though it saves layered and animated files in a proprietary format, if you go into the gallery on your phone these files will be available as PNGs. These PNGs will have their layers and frames separated. This works well for starting a piece on mobile, the bringing it onto the computer to finish.
It does have some quirks. It won't copy/paste anything that is the same color as the background color (easily fixed by changing the background color before copying). Undo can be funny when selections come into play, but there is a histor toolbar that lets you step through any changes made. It also has some other quirks, but overall it's a fantastic program.
Sprite Something (iOS) $4.99
Pixly isn't on iOS, but those with Apple devices do have Sprite Something. This app has many of the same features as Pixly and a great interface as well. I haven't used this one in a while so there might be better alternatives out there. It has a bit of a learning curve for some features, but it is great for making anything pixel-related. Works great on an iPad as well!
Out of the pixel art/sprite apps I've seen and used on iOS, this is one of the best.
Caustic (Windows, Mac, iOS, Android) $9.99 (Android, iOS) Free (Windows, Mac)
Caustic is a nifty audio program/app that has a bunch of features for a good price. It combines a sequencer, mixers, drum pad, vocoder, and various synths that allow you to create music, sound effects, etc. It can sound pretty artificial at times, but then again I'm a crappy musician with no training, so your results may vary.
I like it because you can customize a lot of the instruments, create presets, and the file formats are the same across devices. This lets me start something on my phone, save it to a dropbox, then tweak it on iPad.
There are some better audio apps on iOS like Figure, Rhythm, and Tabletop...but This one works well for what I need.