BugGuide
From the National Audobon Society.
Locate animals by browsing the extensive and image-rich indices for Arachnids, Centipedes, Millipedes, Hexapods, Malacostracans, etc.
Biodiversity Laboratory
Illustrated Guide to Major Groups of Soil Invertebrates
Created by Colby College. A concise, 4-page "cheat sheet", this PDF shows taxonomic organization and illustrations for dozens of invertebrates.
Kwik Key
Kwik-Key to Soil-Dwelling Invertebrates
From NC State University. Identify species by following a no-frills step-by-step guide organized around facets.