Adrenaline Vault Review: Screenshots and opinions, with details of the 'hidden' blooper level.
Garrett's Notebook: Includes a walkthrough which reveals the placement of secrets and loot hidden in the Thief 1 and 2 missions.
Lytha.com: Large sections dedicated to the Thief series of games; including FAQ, newbie hints, surveys, detailled walkthroughs, loot lists and much more about the Thief series.
The Circle of Stone and Shadow: TTLG produced site which includes news, fiction, utilities and a large archive of fan-missions.
The Keep of Metal and Gold: A collection of fan missions with screen shots.
The Keeper Library: Background details, hints and tips about the campaign world.
Thief Underground: Features articles, media, fan missions, tutorials and dromed resources.
Thief: The Dark Site: Comprehensive site dedicated The Dark Project. Guides to loot, maps, background and tips.
Zaccheus - The Honest Thief: Fan missions, links, tips and challenges. Includes dromed advice.