DuSTED is a DS homebrew application, which is a tile editor for GBA and DS hardware-based graphics tiles that actually runs on the DS. It works, but its in a very rough state. It is licensed under the terms of the GPLv3.

devkitARM is required to build it, and homebrew capable DS hardware (or an emulator) is required to run the resulting executable.

http://digitalbytes.net/projects.php#dusted