For the purposes of the MiniDungeons game, I created a number of sprites. The ones that found their way into the current version of the game are available for download below. MiniDungeons sprites by Antonios Liapis are licensed under a Creative Commons Attribution 4.0 International License. Feel free to use, edit and share-alike; if you use these sprites for a game or a research project, I'd be happy to see some attribution (e.g. in the Acknowledgements section of a paper).
Original sprites; transparent pixels are in magenta color.