Maurie,
I often have to examine feather spots and in addition to the Slater Museum website which has proved to be very useful there are two other excellent resources I can recommend.

There is a great book on all types of sign including feathers, called Bird Tracks & signs, here is the link on Amazon:
http://www.amazon.com/Bird-Tracks-Sign-American-Species/dp/0811726967/ref=sr_1_2?ie=UTF8&s=books&qid=1232173844&sr=1-2

An other great online resource is the USFWS Feather atlas, unfortunately doesn't have passerines, but has everything else including lengths of individual feathers. It is probably the best feather atlas I have seen to date:
http://www.lab.fws.gov/featheratlas/index.php

Scott Downes
downess at charter.net
Yakima WA