Links and mirrors
Pan Docs is a comprehensive Gameboy development document written Pan of Anthrox and later maintained by Martin Korth. The document still exists in its original location as well as in the GBDev file hub mirror area. There's also a wikified version on this wiki.
Inside Game Boy Camera
The GameBoy Development Ring
The GameBoy Development Ring is a Web ring with a number of links to Gameboy-related resources. It is a useful resource because it contains mirrors of sites that have gone offline. (Unless that Web space is part of the webring.org service?)
Gameboydev file hub
Gameboy development file hub is a file archive that complements this wiki by hosting files, documents and site mirrors related to Gameboy development. In particular, the file hub has a mirror area with mirrors of most or all pages listed here.