Quick spam blocking question

[Mike Coughlan]

Sorry for the cross post:

If someone has a favorite open source content-analyzing spam filter program,
please send me a link.

We had hoped to implement a dual solution of spam-cop & spam-block, but our
research on other lists on spam blocking has lead to two successful
implementations which were rolled back.

I am hoping this issue has been researched by an IMP admin & open-source
mail guru. Feel  free to respond to off list and to converse via another.
The standard google searches were surprisingly unhelpful.

Thanks

Versions
MTA = Postfix
IMAP = Courier
OS = Linux (Redhat 7.0)
Procmail = Yes