USFWS Feather ID Guide Online

The US Fish & Wildlife Service Forensics Laboratory specializes in identifying protected species for the purposes of prosecution.  As a result, they have a database of the feathers from 213 bird species available online to help in identification.

The online catalog consists of high resolution scans of mostly remiges and retrices (and some body feathers) of many bird species, including a simple key to help find a species based on a found feather.  Thanks to John Gerwin for passing along this great resource!