Stonehenge - Great Trilithon (what remains of) 

Only one of the vertical stones of the Great Trilithon remains.  The stone in the middle of the photo is the the one, and it is the tallest stone at the site.  The little "bump" on the top is the mortise which would have joined with one of the tenons (holes) on the missing horizontal stone.

The structure on the right edge of the photo is one of the trilithons viewed from its side.