Thief 3's modified UnrealEd SDK, originally released in 2005. Read the installation instructions or you'll be sorry!
Ion Storm's UE2-based Flesh engine is incredibly shaky, and the editor is no exception. There are all manner of config tweaks and file overwriting that needs to be juggled when installing and using it.
Guides to using the SDK can be found at the FleshWorks wiki and the TTLG forums.
The Fleshworks Wiki is down, so I used the Wayback Machine to get at the install instructions: Web.archive.org