I thought it interesting that this question is so mainstream that it's even listed on AOL Homework Help and includes the possibility in the article
that it was aliens.
homeworkhelp.aol.com...

Originally posted by Sinobyte
Hundreds of thousands of slaves.
The question of who labored to build them, and why, has long been part of their fascination. Rooted firmly in the popular imagination is the idea that the pyramids were built by slaves serving a merciless pharaoh. This notion of a vast slave class in Egypt originated in Judeo-Christian tradition and has been popularized by Hollywood productions like Cecil B. De Mille’s The Ten Commandments, in which a captive people labor in the scorching sun beneath the whips of pharaoh’s overseers. But graffiti from inside the Giza monuments themselves have long suggested something very different.
... They were not slaves.