You might have missed it yesterday, since it wasn't during the normal Nintendo downloadable game release day, but The Legend of Zelda: Four Swords Anniversary Edition is free to download right now on DSiWare (which is, of course, also accessible with the 3DS). If you've got friends to play with, it's a fun, bite-sized Zelda experience; if you don't, it's basically a really weird Zelda game where you don't get to keep your items.
Another Nintendo classic is available on 3DS today: Super Mario Land 2, which introduces Wario for the first time -- long before he starts his software company. And on WiiWare, escapeVektor: Chapter 1 combines Qix-style outline completion with a storyline about being trapped in a computer.
Nintendo also notes that new 3D trailers for Super Mario 3D Land, Kid Icarus: Uprising, Mario Kart 7, Luigi's Mansion 2, Animal Crossing, Paper Mario, and Mario Tennis will be available on 3DS "soon," so watch all those Blue Man Group videos now to make sure you have time for these.