charliezip
Aseprite is a great tool for creating pixel art and animations.
I've been using Aseprite for many years, now. In fairness, I need to give GraphicsGale and Cosmigo a fair try, and I haven't yet.
Lots of game development tools have decent Aseprite support. More tools should have decent Aseprite support because its file format is simple and the spec is readily available on GitHub.
If you want a jumping off point for writing your own Aseprite file tools, you can find
my little Aseprite loader here:
cd_aseprite.h
That file includes stb_zlib.h, which is Sean Barrett's
public domain zlib decode, extracted from
stb_image.h.
My library does the bare minimum in processing the document data, and it makes lots of tiny allocations, but you should only ever be running it in a debug build anyway!
As an example of what you can do with it, for my last jam game, a build step would suck up Aseprite files, generate spritesheets from them, then "bake" those spritesheets and their animation data as C source code. This allowed me to iterate on the art very quickly.
I don't use many scripts or plugins for Aseprite, but the few I do like come from Kacper Woźniak.