Sobig.E

Discovered on June 25, 2003, the Sobig.E worm spreads via email as an attached ZIP file, allowing it to bypass many filtering products which, by default, focus on executable file types. A backdoor routine leaves infected systems open to further compromise.
Full Story | SoBig a spammer?

Update 09/18/03: Sobig.F worm discovered: Mass-mailing worm far more potent than previous variants.