Congrats from me too. It is true what Ninja Spirit says about the Duo being one of the most niche consoles in the U.S. I guarantee that you won't be disappointed by the sheer amount of innovative, quality games from both the U.S. and Japan.
As for recommendations, I have too many to remember offhand, but I will try my best to list some really great ones.
In no specific order:
HuCards -
Dungeon Explorer - think gauntlet, but with much more depth, more innovations, more freedom, and you have a pretty good picture of this game. My description does not do it justice; you really have to experience it for yourself.
Neutopia I & II - think of the Neutopia as trying to copy about every element of the first zelda game, but adapted to it's own story. Neutopia II; however, strays from the path of just being a zelda clone, but still remains an action RPG.
Tricky Kick - awesome, challenging puzzle game, and no not of the "drop" variety (tetris, puyo puyo, etc.) Although there only a set number of puzzles, will keep you going for a long time, and it is even fun just to play through it again after you have beaten it.
Bonk's Adventure & Bonk's Revenge -
The mascot of the TG-16 before the Duo came out and was replaced with his cousin Air Zonk, was Bonk, and his first two games rocked! Basically a platformer starring bonk a caveman who must battle baddies by "bonking" them with his head. Revenge was very much the same formula with a whole new world to explore. Of course you have to try it for yourself to believe how awesome these two games are. Oh yeah and Bonk III, both in Hu-Card and CD format, is okay but the series was starting to go downhill with this sequel, although it did have the option for two player simultaneous play it just wasn't good enough for me to truly recommend it.
Bloody Wolf - a great action packed game; trust me, just get it.
Blazing Lasers - Awesome vertical scrolling shmup action
CD games -
Dungeon Explorer II - A very good follow up to Dungeon Explorer. Based on the same formula it is a much larger, and longer game with more characters, animated cutscenes and cd audio of both remixes of songs from the first and new ones as well. Although I prefer the chipmusic of the first to the redbook audio of the second anyday.
Lords of Thunder - sequel to Gates of Thunder another great shmup
Beyond Shadowgate - CD sequel to the nes game Shadowgate, but much more awesome. Think adventure, semi-point and click game.
Dynastic Hero - Basically Wonderboy IV, a platformer, (or was it III, I forget) with new character designs, redbook audio, and basically just a big face lift. It's essentially the same game and some people prefer the genesis version(without the major face lift just called Wonderboy IV or III whichever it is). But I love Dynastic Hero, just good luck finding a domestic copy for a reasonable price.
Dracula X - Highly acclaimed and still considered to this day by many the best game in the castlevania series. 'Nuff said.
and too many more that I can't remember offhand.
Be prepared to spend a pretty penny obtaining some of these games as some, like Dynastic Hero domestic version and Dracula X, are EXPENSIVE. But if you do get them you won't regret it.
Happy Gaming shadowlife25!