Overall, I would say that we still don't know for sure, but the general consensus of the experts seems to be that the labor force that built the pyramids were not free wage workers in the modern sense, but there is no evidence to suggest they were chattel slaves to any significant extent.
Here is a nice short article, "Who built the pyramids" from Harvard Magazine, highlighting the work of Egyptologist Mark Lehner who excavated the facilities where pyramid-building laborers lived.
It describes various bits of evidence that workers were probably not slaves, such as the fact that they seemed to have been eating large quantities of meat. But I think the most significant point might be this:
Lehner’s friend Zahi Hawass, secretary general of the Supreme Council of Antiquities, who has been excavating a “workers’ cemetery” just above Lehner’s city on the plateau, sees forensic evidence in the remains of those buried there that pyramid building was hazardous business. Why would anyone choose to perform such hard labor? The answer, says Lehner, lies in understanding obligatory labor in the premodern world. “People were not atomized, separate, individuals with the political and economic freedom that we take for granted. Obligatory labor ranges from slavery all the way to, say, the Amish, where you have elders and a strong sense of community obligations, and a barn raising is a religious event and a feasting event. If you are a young man in a traditional setting like that, you may not have a choice.” Plug that into the pyramid context, says Lehner, “and you have to say, ‘This is a hell of a barn!’”
Lehner currently thinks Egyptian society was organized somewhat like a feudal system, in which almost everyone owed service to a lord. The Egyptians called this “bak.” Everybody owed bak of some kind to people above them in the social hierarchy. “But it doesn’t really work as a word for slavery,” he says. “Even the highest officials owed bak.”
The article concludes with the disclaimer that this hypothesis is still subject to peer review and further research. That said, I can find no solid reason to doubt the following, reported in US News and World Report:
Dieter Wildung, a former director of Berlin's Egyptian Museum, said it is "common knowledge in serious Egyptology" that the pyramid builders were not slaves and that the construction of the pyramids and the story of the Israelites in Egypt were separated by hundreds of years.
"The myth of the slaves building pyramids is only the stuff of tabloids and Hollywood," Wildung told The Associated Press by telephone. "The world simply could not believe the pyramids were build without oppression and forced labor, but out of loyalty to the pharaohs."