If you don't have Petit Computer already, you should get it now. Don't believe me? Look at this.
Petit Computer - $8
What you get from it:
MegaMan 2 PTC (Megaman 2, $5)
QuikNote (myNotebook things, $5)
Petitcom Music Workstation (too many to list, ~$5)
Sample game 1 (Dotman, $2)
Petit Tetris (Tetris, $2)
Breakout (many, $2)
That's $23 worth of eShop content, not to mention all the exclusive Programs we have, and versions of existing games (Minesweeper, anyone?). Add on the ability for you to make your own programs and share them, and you wonder why everyone with a DSi or 3DS doesn't have this. If that wasn't enough to convince you, it comes with a biorythym program (2 dollars?), mathematical functions, and there are editing programs for a computer that you can use to program on your computer, then test on DS.