I have a vps that does nothing but handle mail for 1 account, I needed something to block spam, but running spamd was too heavy for my vps. I run this every minute. it simply finds every mail <2 minutes old and moves it if spam assassin claims it is spam. It's low tech, but also low requirements.

spam.sh

#!/bin/sh
INBOX="/path/to/Maildir/cur"
SPAM="/path/to/Maildir/.spam/cur"
cd  $INBOX
for i in `find ./ -type f -mmin -2`
do
        echo "Testing $i"
         if (cat $i | spamassassin | grep  'X-Spam-Flag..YES')
                then echo "$i is spam"
                mv $i $SPAM
        fi
done