Anti-spam products tend to vary enormously because they all use different approaches -- some use weighted Bayesian algorithms, which are becoming recognized as the best way to fight this sort of thing, but they require training. One of the best programs I've dealt with is actually an end-user program called MailWasher. MailWasher can work with a centralized list of recognized spammers, which could be set up on a network/shared drive and administered centrally. For anti-spam solutions in a domain, MailCleaner is another solution and doesn't require any installation on the end user's computer.
This was first published in January 2003