I realized I've been tinkering around with this enough I ought to treat myself to an Arduboy FX before they sell out this year, so I snagged one. This is a very neat little handheld with good build quality and buttons, and it is far more enjoyable to use than my devboard. It has a battery life of about six hours and it contains a W25Q128 flash memory module, which is a lot bigger than the W25Q32 I picked up for my own tinkering a while back. The only thing I missed was a volume control button. Most games have an option to toggle sound, but not all.
The FX comes with 200 preloaded games, but you can add more using randomouscrap98's Arduboy Toolset (haloopdy has written a guide) or use poevoid's guide, which requires Arduboy Python Utilities and Arduboy-FX-modchip.
Source files can be exported to Intel HEX format via the Arduino IDE. Erwin explains it all here.
There are interesting tools and utilities:
A few games I found that aren't pre-loaded...