Fun fact: GBA emulation and homebrew development actually started before the system's release [1]. IIRC, the official devkit was leaked, then someone wrote a summary of the low-level documentation and started circulating that, which (since the CPU had an off-the-shelf ARM core) was enough to start making rudimentary emulators and tech demos.

[1] https://web.archive.org/web/20001214040900/http://www.zophar...