- Unsolicited commercial email. The scourge of the internet and probably the largest downfall on the Internet seconded only by popups. Spam has almost made email as a method of communication useless.
- A popular luncheon meat that is made with, well, nobody is quite sure.
One of the people responsible for the GPL sanctioned spam  -- and then later backpedaled.
The first real spam was Canter & Siegel who posted an Immigration pitch letter to thousands of newsgroups. As much as the general internet population hated it, they did get enough response out of it to deem it a success, and later wrote a book and probably were most directly responsible for the rapid growth of spam on the internet.