http://spamassassin.apache.org/
This is the only one that I know someone who uses a spam filter and they like it.
For myself, what I do now is forward all my email to a gmail account and either use the web interface, or setup the email client to use IMAP to look at the gmail mailbox.
This has the benefits of Google's spam filter which has been very very good, and also allows me to not have to worry about having 3 different places my emails being downloaded and having messages be read / unread here or at home or whatever.. plus I can access and search through all historical emails anywhere and fast.
Plus works well with the iPhone.