I have a major spam problem at work. I get a ton of it. Recently I discovered Spammunition - a free Outlook add-in that uses a Bayesian filtering system to catch spam. It rocks - great filtering, very low false positive rate. It's still pretty early beta stuff, but I couldn't go back to life without it. Highly recommended.
http://www.upserve.com/spammunition/default.asp
Mini-rant: What's the deal with Hotmail? I have NEVER used my hotmail account for any emailing of any kind (I just got it for MSN Messenger), and yet I get loads of spam there. That pretty much guarantees that I never will use it for email. Microsoft, you may want to work on your spam filtering engine (or just license Upserve's code). Neither my Yahoo nor my Earthlink accounts have anywhere near the spam problem that Hotmail has, and I actually give those email addresses out.