Getting Semantic With Microformats, Part 1: rel ~ A Blog Not Limited
A great article by Emily Lewis on the microformats that make use of the rel attribute (rel-tag, rel-license, XFN, etc.).
FUMSI Article: Microformats: Digging Deeper into the Web
A nice, neat, short introduction to microformats from Ben.
Related links
Microformats: More Meaning from Your Markup [HTML & XHTML Tutorials]
Great article from Brian ranging from introducing microformats right up to the current state of play.