Dichotomous key for identifying / discovering LEGO pieces?
Closest thing that I know of is the peeron site - you can use google to search only peeron by adding "site:peeron.com" or you can use the peeron website to find the part.  Start by looking at the part list for a few sets you have (or see for sale) and then when you get used to the terminology you should know how to search for the unknown part.  It lists all sets/years that the part came from, and all colors it is available in, and uses Lego part numbers and set numbers (it ads an extra digit to set numbers that have been reused by Lego)

brickwiki says: 

Peeron is a set database with set inventories and an instruction scan repository (PICSL1) run by Dan Boger, with the help of a number of volunteers (Clark Stephens, TWS Garrison and a number of developers). 

[ . . . ]

Peeron is one of the three major sources (with BrickLink and LDraw.org) for nomenclature and classification, it maintains a color list, and assigns (in cooperation with LDraw) names and numbers for parts that haven't yet been named.