The Academic Site of Velonis I. Petros

In Windows you can use SpamPal which is a POP3 Proxy Spam Filter. I installed it to one of my clients and the results are very good. It can be used for either Windows or Linux.

Either you use Windows or Linux you can use Sylpheed with BSFilter. They are both compiled for many platforms. 

Some filters integrating to Outlook are

• Inbox Protector (some problems with foreign language Spam)
  
• Spam Bully 
• SpamArrow
• Spartacus Filter
  

There is a very good site, it is SpamLinks, where you can find

• Integrated Spam Filters for Windows: Outlook, Eudora...
• Webmail Spam Filters: AOL, MSN, Hotmail...
• Windows POP3 Spam Filters
• Windows IMAP Spam Filters
• Other Windows Spam Filters