Actually SpamAssassin runs under *NIX but there is a Outlook version.

Yeah, I run SpamAssassin via procmail on my Unix box. It works a treat. Very few false positives (just a few mailing list posts that I'm not too bothered about). There have been a few false negatives, but it's cut my spam by quite a lot.

I'm generally happy with it, but I might install TMDA as well.