The most widely accepted system is Mitchel Lichtman's UZR.
It was initially introduced in 2003 in twoarticles over at Baseball Primer.
UZR numbers for 2000-2003 were then posted on TangoTiger's
website, where they can still be found.
Complete numbers for 2004 and 2005 are not available, as Lichtman was hired by the St. Louis Cardinals and his work became proprietary data for their club.
has released some UZR numbers for the past two years in different discussion threads at Baseball Think Factory
, but for the most part, current UZR data is no longer public information.
David Gassko's way to tell if his system was "working" or not was to compare correlation to UZR, and Mitchel Lichtman
gave him enough data to compare between the two.