UX.Unleaked
UX.Unleaked is a blog by Grabberslasher that has documented several unleaked Longhorn builds during the time (2008), such as 4030.lab06_n.030630-1724, 4042 (main compile and lab06 as both had not been shared at the time the post was done), 4040, 4084, 4002, 5098, 3790.1232, 5001, and 4066.
He also had plans to document builds 4030.main, 4031.main, 4031.lab06, 4032 and 4038 (some SKUs) but never documented them.
This blog has no new posts from the owner since 2 July 2008. As of today, all of the builds listed on the site have been shared, mostly by Grabberslasher himself.