Rgbds - Rednex Gameboy Development System Documentation in html.
gbspec - the major document about regular gb stuff. aka the 'pan document'
cpu timing - rough timing overview
memory map - general memory map
display timing - display timing diagram
display interrupts - how to use most common interrupts
dma transfers - how to use dma transfers
opcodes - 8080/z80/6502 opcodes comparison table