Theories on how the Great Pyramid at Giza take various forms. Most of them have involved some form of ramp, up which the large stone blocks were hauled. The problem was that construction of some of the ramps would have taken more effort and material than the pyramid itself.
Recently a French architect has proposed a theory that seems pretty logical, and solved a lot of the problems with the previous ramp theories. Fleshing out a theory suggested by his architect father, Jean-Pierre Houdin spent most of a decade planning how he would build a pyramid, and developing 3D computer models to show how it could be done. His plan involves the use of a ramp, but one which is internal to the pyramid, which is made of straight sections which turn at the corners and create a straight sided spiral to the top of the pyramid.
When the ramp reached a corner, there would be a platform and a crane, for turning the block of stone 90 degrees, so it could proceed up the next section of ramp.
Of course, the ramp theory will not be proven until a pyramid is taken apart to verify that an internal ramp exists, which is not likely to happen anytime soon. Mr. Houdin was resigned to that uncertainty, and had been making presentations of his theory. After one of his presentation, he was approached by a member of a German team which in 1986 had undertaken a project of scanning the pyramid looking for internal passages and chambers, using a sensitive gravity sensing technology. The member of the German team said that beside their published report, they had some images which they could not explain, so they were not included in the report. The images are below, and show regions inside the pyramid which showed as low density zones. Maybe they are internal tunnels, ramps, or tunnels filled with rubble from the building process. They match almost exactly Houdin's calculated positions of the internal tunnels and ramps.
Another bit of supporting evidence is seen on the pyramid itself, where a notch on one of the corners appears to be an exposed turning platform theorized by Mr. Houdin.
When the notch was explored by Bob Brier, he found a small room behind the platform, inside the pyramid, on the notch. This room appeared to be from the time of the construction of the pyramid, because the entrance to the room was smaller than the stones that would have been removed to form the room later. The roof of the room also had stones which were shaped to a dome shape, to support the weight of the stone above the room. A project has been proposed to penetrate the wall of the hidden room to see if behind it is an internal ramp.
This theory was presented by Mr. Houdin and Bob Brier in Secret of the Great Pyramid. See these links for more information.