If you're willing to compile it, Aseprite is a great option as well.
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Game Textures has recently updated its vast library with 150 new material substances.
All the materials are tillable, tweakable and could be used in games, scenes, or just to test some stuff. Mostly the artists are to choose from a variety of various tiles with some very pretty geometry. There are clean materials, which will work in architectural visualization, but there are also nice cracked textures for your post-apocalyptic projects.
Check the full list here at Game Textures.