https://help.ubuntu.com/community/Grub2

  - Power up your computer.
  - Press **SHIFT** at the grub menu.
  - Highlight the line that begins “Ubuntu, KERNEL ………” and press **e**.
  - Press **e** for edit.
  - Highlight the line that begins “linux ...”
  - Go to the end of this line, add **single**
  - Press **Ctrl-x** to boot your system.
  - Your system will boot up to a passwordless root shell.
  - Type in **passwd username** where username is the username you lost the password for.
  - Set your password.
  - Type in reboot.

http://freebsdwiki.org/index.php/GRUB_FreeBSD

root@gate:~# cat /boot/grub/menu.lst 
default saved
timeout	10

title           FreeBSD
root (hd0,0,a)
kernel /boot/loader
savedefault

title		Ubuntu
root		(hd0,1)
kernel		/boot/vmlinuz-2.6.24-16-server root=UUID=08931346-445f-4a43-b136-dcb96397b8c6 ro quiet splash
initrd		/boot/initrd.img-2.6.24-16-server
savedefault

title           Windows NT/2000/XP
root (hd0,2)
savedefault
makeactive
chainloader +1

# grub-set-default 1