Orbit 6 - Space Technology and Fiction:Includes links regarding the proposed usage of ET, and technical papers.
External Tank Manual: Features extract from the shuttle reference manual.
PERUN - Space Shuttle External Tank Used as a Space Station: Paper written in 1979 deals with the possibility of converting an expended Space Shuttle External Tank into a Space Station.
Re-using today's rockets spent fuel tanks in orbit: Technical website
Space Island Group Project: Features a project descriptions for building a space station devoted to tourism and 0-G experiments, based on the tanks of the space shuttle. Regularly updated.
Space Shuttle External Tank Applications: Overview of possible applications of Space Shuttle External Tank
Space Shuttle External Tank Applications: Offers a collection of articles arguing for several applications and re-use of the tank.
Studies: Studies by L5 Minnesota (MN Space Frontier Society)- Uses for Shuttle External Tank in orbit