This is bootloader, which can boot custom boot image even droid-family phone has locked bootloader.
It consist from:
- small kernel module, for creating device for booting/controlling boot
- small userspace program, which give for module boot image and flags
- universal bootloader, which can uses many places for booting
It's derived from collaborative work of yakk and dimichxp for creating bootloader for older motorola phones, before their RSA have been cracked.
Now, this project ported to the Milestone hardware and can boot custom kernel fully, except of baseband part.
Here you can find current development sources of 2ndboot: 
Here you can fin howto build 2ndboot from sources: Compiling
Also it is necesseary for developing 2ndboot and custom bootloader/kernel to debug over serial port: Debugging