10. Morris (1988) -- An oldie but a goodie; without Morris the current threat "superstars" wouldn't exist. The Morris worm (or Internet worm) was created with innocent intentions. Robert Morris claims that he wrote the worm in an effort to gauge the size of the Internet. Unfortunately, the worm contained an error that caused it to infect computers multiple times, creating a denial of service.
To round things out, Symantec offers a list of all the top threats, though you don't necessarily need Symantec software to protect yourself. Security experts say no one program will protect you against everything, and some of the best malware protection can be downloaded for free. (Take a look at AVG, Malwarebytes, or Spybot as a few examples.)
So there you are. Feeling nostalgic? Or are you still trying to get the Morris worm off your hard drive?