Currently I'm using TileStudio for that, but since it's quite old, it doesn't go well with my 1080p resolution. Any other apps that are free (not "demo" kind of free like that PyxelEdit or whatever it is called, but actually free) and have facilities to easily pixel out tiles, with things like e.g. looping preview or ability to test out tileset in rudimentary map editor. Basically TileStudio but better.
//edit: Nvm, found my pick. Pixothello.