Outtakes - scraps of material collected from Hugo's notes and rough drafts, deleted scenes if you will. All the best bits (including an unexpected meeting between Patron-Minette and the Friends of the ABC, and a fanfiction-esque interlude about what happened to Tholomyès) have been translated into English, and the rest (including a strange digression on prostitution) is slowly in the course of being translated.
History - Bits and pieces about 1820s and 30s France. A timeline, modern equivalents of currency and units of measure, excerpts of letters related to the events in the book, a lot on 1830s republicanism (inluding some publications of the Society of the Friends of the People, loosely the model for the Friends of the ABC). Mostly translated into English, except the longer Friends of the People material.
Paris - Digitized maps of pre-Haussmann Paris, some links to websites on Parisian cartography, a few early 20th century photographs of the Rue Mondétour.
The Mizzie Tour Guide - Finding the places in modern-day France (particularly Paris) where the action of the novel took place. Photos of Montreuil-sur-Mer, the site of the barricade, the part of the Seine where Javert drowned himself, etc.
Cast recordings - Cast lists and distribution info for all the major cast recordings of the musical. (Yes, I know it's three years out of date.)
Translations of the French lyrics - An attempt at a literal translation of the lyrics to the 1991 Paris revival version of the musical.