Terraserver-rrrip!
t s z
low-X high-X
low-Y high-Y
Example locations:
Coolfont, WV
Sodus Point, NY
Richmond, VA
(The area around my apartment, in Richmond)

Coolfont, WV (huge topographic map!)
Coolfont, WV (huge aerial photo!)
Letchworth Gorge, NY (huge aerial photo)
Bryce Resort (includes "Sky Bryce")