I recently finished making Solitaire for the Nintendo E-Reader. I managed to fit it onto a single card, and it's a pretty full featured version of the game. I'm really happy with how it turned out. I figured I'd talk a bit about how I made it in what turned out to be a long blog post :)
The E-Reader is a Game Boy Advance peripheral that Nintendo released in 2002. By scanning cards that have a dot code strip on them, you can load mini games, extra levels, animations and more.
I've always really liked the E-Reader and was sad it didn't do too well in America. So I thought maybe I'd take a stab at making games for it myself.
Where to begin? I remembered there were some old tools and websites about making E-Reader cards from back around when it first came out — twenty years ago! I managed to find Tim Schuerewegen's original site in the Wayback Machine. It had some examples, source code and tools. I also refound CaitSith2's E-Reader site, which thankfully is still up. It also has some tools and information.
These initial findings were a great start and got me headed down learning how E-Reader applications are programmed. GBATEK also has a section on the E-Reader which also contains lots of useful information.