Cloudmark is the best of all the solutions I've tried or read about it. It really is amazing and the concept is much better than the others out there. It's community based so you tag stuff that slips through (not that many) as SPAM and then next time someone else gets it, it'll already be detected.
I just installed the trial version, and (so far) it seems to be working much better than anything else I have tried for spam. Thanks for posting the tip!
I am also trying NOD32. It seems that it`s a very light load on system resources compared to other software, but I`m not sure how good it is at blocking viruses and spyware yet.