The DS was pretty popular for homebrew; the microSD card adapter cartridges were impressive feats of engineering which made it very easy to load custom code. I remember ebook and graphic novel readers, paint apps, Linux, emulators for a few simple systems like the SNES, a C SDK, the whole nine yards. It wasn't as fast as the PSP, but there were still some pretty amazing projects, like this homage to Portal:
https://web.archive.org/web/20160203155724/http://smealum.ne...
Sad to see that link rot is starting to set in with many of the projects that I mentioned above. For example, the SNES emulator: