I'm using Pixel Studio. It's free, easy to use and you can get it on pc and android. You can even make animations and play them while working on them, and save them as sprite sheets when you are done. I don't recommend Gimp, because it takes way to long to load, and most of the features are unnecessary for making a texture pack with a resolution below 64x64. (English isn't my native language, so sorry for any mistakes.)
