I've had an R4 card for a while but really haven't used it for much else than playing DS games and running ScummVM few times. Today I found out about a cool little app called Treasures Of Gaia which is basically Google Earth for your DS:
http://stravingo.over-blog.fr/article-11878935.html
It's not perfect but I was really amazed with the level of quality it had for a homebrew app. Was wondering if there are any other good non-emulator homebrew apps that I need to know about. I hear a lot about the ones for PSP but hardly nothing for the DS.