GCF Learn Free is a website that offers free tutorials, both in text and video format. People often ask me where they can learn PowerPoint from scratch, and this is a good place to start for the basics. Go to the Computer Training page, choose PowerPoint, and choose the version you want. They have tutorials from 2000 through 2007 (PC only).
If you try these, let me know what you think. Do you know of other free PowerPoint tutorials?