We all get too much spam. Here's one way to stop almost all of it. We are going to use Gmail's spam filtering without being stuck with the gmail interface. All it requires is a gmail account (who doesn't have one of these?) and an unused email address (that doesn't already get spam). Who knows how long this will work for seeing that gmail is beta, but it works so well I am willing to chance it.
Step 1: Setup Your Gmail Account
We'll assume that you have a GMail account and you know how to log into it. So, log in and go to "Settings". Click on the tab "Forwarding and POP". Select the radio button next to "Forward a copy of incoming mail to" and enter your unused email address. Then choose "Trash Gmail's Copy" so this account's inbox doesn't fill up.
Note: Never use the email address that you entered for anything else. It should remain secret and away from direct spamming.
Step 2: Now Forward Your Spam Ridden Email Address to Your Gmail Account.
Now forward your spam ridden email address to your gmail account. Most mail providers give you a way to do this.
Step 3: Set Up Your Mail Client to Just Check the "secret" Email Address
This step will depend on your mail client. It's conceivable that you could just use gmail as your client, but I don't like it's functionality. So, instead I use Mozilla's Thunderbird.