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When this is complete, it will contain information about and links to web sites of other spam filters. The information may include a product review, if I think a particular filter is unusually good or unusually bad, or might be useful in a particular circumstance.
While I naturally prefer the SpamBouncer 
, there are a number of other good spam filters available to users. I list a few of the better ones below. The ones at top of each list and in boldface are filters I have tested and recommend as "best of class/best of breed" for their niches.
NOTE: The products I list are free unless marked with a $ at the beginning of the link. Some of the commercial filters have free trial periods, however, so it may be worth while to install and test them.
For low-volume Unix servers and Unix workstations, there are a number of decent choices for spam filters. SpamAssassin a good choice if you don't mind a fairly complex installation and configuration procedure. It is both fast and effective. Other filters are easier to install and use on Unix shell accounts, however, and may catch more spam. Most Unix client and low-end server filters are free, so try them out and see which works best for you.
The spam filters listed below are popular spam filters for Macintosh users. I have not worked with or used any of them personally, and none stand out above the others for me, so I don't make a recommendation. I have reviewed the Windows version of PopFile, and it works quite well, but the cross-platform version is based on Perl and probably requires more than the average Mac user's Unix command line skills to install and configure. I would guess that SpamSieve is probably the closest Mac-native equivalent to the Windows PopFile. If it works as well, most users will be happy with the results.
NOTE: Macintosh users running OSX who don't mind installing and configuring a spam filter at the command line should also check under Unix filters. Macintosh OSX is Unix beneath the GUI and can run most Unix command-line software.
Because of the profusion of new spam filters for Windows, I've restricted this list to a few products that I have personally tested and know work well. Of those, The PopFile filter is, in my opinion, the easiest to install and overall the best. However, I last tested it a couple of months ago, and new and improved products are popping up daily in this field. As long as you have secured your Windows computer properly and get your anti-spam programs from trusted sites (like these), feel free to try out different ones to see which works best.
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For completeness, I list other options, but if you have a Unix server handling more than a couple hundred email accounts, there is one clear stand-out among spam filtering programs -- SpamAssassin. If your mail server handles email for more than a couple hundred people and under a couple thousand, don't even look further. It's the best, and it's free. (Yes, better than the SpamBouncer for this use. D*mmit!)
Microsoft Exchange 2003 Server has a number of built-in anti-spam features, including the ability to check DNS-based blocklists and whitelists at SMTP transaction time. There are useful spam filters that work on Exchange servers, however, and you might want to install one or more of them in addition to using the built-in features. I have not administered an Exchange server, and therefore am not recommending any specific product, although I will note that the Exchange Intelligent Message Filter from Microsoft is free and the others are not.
NOTE: If you are using a Microsoft Exchange Server prior to Exchange 2003 to handle your company's email, your best solution is to upgrade to Exchange 2003, replace it with a Unix-based email server, or firewall it and place it behind a Unix-based server that accepts incoming email, filters it for viruses, trojans and spam, and then passes it on to the Exchange server. Earlier versions of the Microsoft Exchange Server have a large number of security vulnerabilities; you should not risk using them on the open Internet.
If your mail server handles mail for more than a couple thousand people, I believe that the Symantec Mail Security 8100 and 8200 series appliances are best-of-breed. They both incorporate the technology Symantec acquired when it bought Turntide in summer 2004. TurnTide developed a unique, patented approach it called "traffic shaping" to detect and throttle high-volume spam streams, allowing legitimate email to get through. This technology allows the Symantec servers to detect and stop spam sources quickly without delaying email delivery or blocking any legitimate email whatsoever.
In addition to the Turntide technology, these servers also incorporate technology Symantec acquired when it bought Brightmail, and integrate other spam filtering technology from other sources. The result is what appear to me to be a formidably effective line of spam blocking firewalls.
My second choice is the Barracuda Anti-Spam Firewall. It is a decent product and they are working hard to make it better. If you use it, however, be sure to turn off bounce generation; Barracuda firewalls in their default configuration have a tendency to create "blowback spam" during high-volume email virus infestations. Barracuda really does need to change their default settings; I hear that they are aware of this and doing so in their next software release.
Symantec's Brightmail is by far the best of the outsourced spam filtering services I know about. Brightmail combines a patented (and secret) set of automated spam filters with human review, resulting in fairly effective spam filtering with a very low false positive rate. I have tested Brightmail, both some years ago when it was new and recently, using a "trial period" license, and both times found the results acceptable, although more spam slipped through than I like.
Brightmail is quite expensive. A small- or medium-sized company that lacks the resources to handle its own server-based spam filters in-house, however, might well find the expense worthwhile.
Postini is a less expensive solution than Brightmail. It also provides fairly effective spam filtering, but with a considerably higher false positive rate that I attribute to Postini skipping the "human review" of filtered email. The company I work at uses Postini, and I have heard numerous complaints about important business email being tagged as spam. Postini also has a well-deserved reputation for slow and unresponsive customer service, however, which turns what could be a manageable false positive problem into a much bigger deal. I don't recommend Postini.
I don't know enough about the other companies that offer this service to have an opinion.