A spam filter that is a plug-in to a specific mail-reading program (or is a description of how to write filtering rules for that program) belongs in the category for that mail reading program. (For example, spam filters for Eudora belong in Computers/Software/Internet/Clients/Mail/Eudora.)
A spam filter that is a standalone application meant to be run by a client should go to Computers/Software/Internet/Clients/Mail/Windows/Tools/Anti-Spam (if it is a Windows program), or Computers/Software/Internet/Clients/Mail/ if it runs on any other operating system.
Spam filters that work with only one type of mail server should go in the category for that particular server; for example, Sendmail filter rules belong in Computers/Software/Internet/Servers/Mail/Sendmail. There are similar categories for a variety of mail servers in Computers/Software/Internet/Servers/Mail. If there is no category for the mail server your spam filters are written for, you can submit them here. Procmail filters go in Computers/Software/Internet/Clients/Mail/Unix/Procmail whether or not you think procmail is a client.