These are revised textures for the Shuttle 3.5 series. They change Challenger, Columbia, old Columbia and Endeavour to add cargo bay vents on the sides of the fuselage, and black tiles on the front of the OMS pods.
I realize the vents are only open during launch and landing, but they definitely make the shuttle look more realistic, for you purists out there.
These are based on actual shuttle launch/landing photographs. This was a joint effort; I (MrBatman) did the textures and Melanie (Terra Nova) did the research.
Slightly revised: Melly pointed out a couple of small texture errors, which I corrected. (A vent was misplaced.)