The Nexus One ROM has been copied and modified to work on the Droid. What does this mean exactly? Well, the Nexus One utilizes the Android 2.1 OS which is newer then what we currently have for the Droid.
If you are feeling particularly bleeding edge and daring you can attempt to put this on your phone, but beware, it's not for the faint of heart. Already there are reports about some people are having difficulties.
If you wish to continue please do a complete backup first and be aware that you could brick your phone, which won't be covered by your warranty.
The following thread has more information on the process (Thanks LosAngeles):
Apparent stable 2.1 ROM for droid