A French architect named Jean-Pierre Houdin has used 3D modeling software, combined with rigorous study of the pyramid itself, to advance a startling new theory that supported by some pretty compelling evidence.
Over at Archaeology, Bob Brier explains Houdin's theory. Basically, he suggests that the base of the pyramid was constructed using a ramp that was later dismantled and used to build a winding, spiral ramp on the inside walls of the pyramid. People carried stones up these ramps, building more ramp as they went. When they finished, the ramp was sealed within the pyramid walls. Writes Brier:
The architect has concluded that a ramp was indeed used to raise the blocks to the top, and that the ramp still exists—inside the pyramid!
It may sound a little far-fetched, but Houdin points out that not only do internal structures in the pyramid match his descriptions, but an image taken of the pyramid's structure in the 1980s also backs up his theory:
At left, you can see one of the microgravimetry images showing a structure that does look very much like a ramp hugging the inside walls of the pyramid.