GRUB "Legacy"

• manual
• official wiki
• list of commands
• GNU GRUB (0.97) Simplified for Newbies
• smaller articles
  • GRUB Splash Image HOWTO
  • pre-made boot images: [1] [2] [3]
  • Gentoo's GRUB documentation
  • Linux Journal article
  • Sun article
  • Red Hat Magazine article
  • O'Reilly article

GRUB4DOS

• see README_GRUB4DOS.txt from the official distribution, it contains the most official information
• diddy's GRUB4DOS Guide is also quite lengthy
• official wiki
• new commands:
  • several map changes
  • find --set-root
  • cdrom
  • setvbe
  • write

Super Grub Disk

• official wiki
• new commands:
  • usbshift
  • setgrubdevice
  • grub_add
  • findf

NeroGrub

• official documentation
• official wiki
• blog

GRUB 2

• official wiki
• new commands