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: 
Also it is necessary for developing 2ndboot and custom bootloader/kernel to debug over serial port: Debugging
- You need kernel sources for that from Motorola
- You need special arm toolchain for that
- Build you module of 2ndboot
You can find all info about compilation of kernel, toolchain and 2ndboot here
Latest attempt: Yakk patched 2nd-boot to make it work on the Milestone, but his source code remains partly unpublished at this time. Status: boots another kernel, which Yakk has also patched to get serial output over the USB connector (using custom hardware to connect to it). Currently the booted kernel has some problems with USB and fails to initialize the phone's modem so it crashes. See published code and binaries: here (build number 1.03) and here (build number 2.31). All current development of 2ndboot now going here When GSM is disabled this kexec module is able to boot the system with the recompiled kernel, but it is not really useful as a phone then. WiFi works fine, though. Yakk is now trying to use 2ndboot to start a patched version of mbm, which should be able to initialize the modem and then pass control to a custom Linux kernel. This is still under development, so don't get too excited. We'll keep you posted.
mount -t debugfs none_debugs /sys/kernel/debug cat /sys/kernel/debug/usbmon/1t