While not fitting exactly in the spam category, it seems appropriate to mention virus and worm attacks in the same context. If nothing else, they are just as annoying, or even more.
The exact difference between a virus and a worm is somewhat fuzzy, but they are both malicious in nature, and propagate automatically when their victims activate executable attachments to email messages.
They also make the infected messages rather big, which will signifiicantly increase the time it takes to download your email via slow modem links, so you might want to use procmail to filter these out at the time the mailserver receives them.
Trojan (as in Trojan horse) is another piece of
trickery, that you might get exposed to if you read your email with
a program that understands
or visit webpages containing online forms.
Do not give out personal information, such as login name, passwords or credit card numbers if you aren't absolutely sure, that you are looking at a genuine document!
Spam Control at Nyx