There's also the Nintendo Channel, which provides demos for download, but only if you agree to send your play statistics to Nintendo. Sometimes there are decent games on there, like Big Bang Mini recently, which was up as a demo before its release date. A few games also include DS demos: Geometry Wars: Galaxies is one (you can also unlock an extra galaxy if you have the commercial DS game), and MaBoShi/Maboshi's Arcade offers a demo, too.
EDIT: There's something to do with Pokémon, too, but I really don't keep up with that stuff.