There's a whole collection of great DS games.
I'll list em.
Animal Crossing,
Bionicle Heroes,
Bomberman Land Touch! (one and two)
Chrono Trigger,
Custom Robo Arena,
Drawn to Life and it's sequel "The Next Chapter,"
Final Fantasy III and IV
Kingdom Hearts 358-2 Days (not too shabby)
Kingdom Hearts Re-coded
Kirby - Squeak Squad
Kirby - Super Star Ultra
LEGO Star Wars - The Complete Saga (Graphically unappealing, but hey, it's a DS)
Mario Kart DS
All of the Mega Man Star Force games,
Mega Man ZX and ZX Advent,
Mega Man Zero Collection,
Metroid Prime Hunters,
N Plus (N+),
Need for Speed - Undercover (if you want some racing action)
Phantasy Star 0 (personal favorite)
POKEYMAN GAEMZ
Sonic Chronicles, Sonic Rush, Sonic Rush Adventure,
Star Fox Command,
Super Mario 64 DS,
Super Scribblenauts,
The World Ends With You (I hear it's not common in stores)
Transformers Autobots and Decepticons (the original ones on the DS, none of the newer ones)
Worms - Open Warfare 2
Zelda Phantom Hourglass and Zelda Spirit Tracks.
And there you have it. All of the games that are worth playing on the NDS.
(In alphabetical order (more or less) too!)
