Information about MLM (multi-level marketing) and chain letters from the U.S. Postal Inspection Service
The Federal Trade Commission has educational materials on E-Commerce and the Internet , an older article on pyramid schemes, and a good privacy issues page -- definitely some words to the wise!
I've been told you can send MLMs and ponzi schemes to fraud@uspis.gov, pyramid@ftc.gov, fraudinfo@psinet.com and the last I heard the IRS couldn't do anything about these via email. "Regular" UCE goes to uce@ftc.gov