Once a month I get one or two pieces of spam in my inbox. So far no "normal" email has been misstaken for spam as far as I know.
The software is called SpamBayes.
I run it as a proxy. That means that my email client (Thunderbird from the Mozilla project) connects to
localhost which gets forwarded to
mail.grenna.net which is the real mail server.
The software holds absolutely no information about what is "spam" and what is "ham" initially, so you have to train your SpamBayes for a couple of days using an easy web interface to say: "This email is spam", "This email is ham", ... Then SpamBayes breaks down each word in the email and can tally each word with a probability-of-being-spam. Don't worry, it's much less complicated than I fail to explain it as.
Read this article which is another success story like this.
You will need some computer skills to set it up, but if you use M$ Outlook you can download an installer that should be more or less double-clickable.
It doesn't work with hotmail, yahoo or alike but if you have a POP3 account and hate spam I strongly suggest you have a look at this.