Statistics and log analyzers
These are tools to analyze SpamAssassin logs and generate reports. Generally, they take syslog data logged by the 'spamd' server as input.
qmailmrtg [http://www.inter7.com/mrtg.html] - example output: [http://www.sk-branik.si/qmailmrtg]
vispan [http://www.while.homeunix.net/mailstats/] - demo: [http://hosting.boys-brigade.org.uk/mrtg]
mailwatch [http://mailwatch.sourceforge.net/] - no demo, but the previous link has screenshots
mailscanner-mrtg [http://mailscannermrtg.sourceforge.net/mailscanner-mrtg/] demo: [http://mailscannermrtg.sourceforge.net/mailscanner-mrtg/]
logreport.org apparently has a syslog analyzer that can grok spamassassin logs: [http://logreport.org/lire/or/email.php]
sa-exim-stats [http://nossie.addicts.nl/projects/sa-exim-stats/] : Generates rrdtool graphs from sa-exim log files.
mailgraph [http://people.ee.ethz.ch/~dws/software/mailgraph/]: Generates graphs from syslogs. demo: [http://people.ee.ethz.ch/~dws/mailgraph/mailgraph.cgi]
monkeynoodle.org spamassassin MRTG scripts [http://www.monkeynoodle.org/comp/tools/spam]: Scans the maillog/syslog and feeds the SA results into a format from which MRTG is used to create graphs. demo: [http://www.monkeynoodle.org/mrtg/felix.html]
spamdstats [http://www.ini.ethz.ch/~stephan/software/spamdstats.tar.bz2]: Continuously parses spamd's log file in the background and stores various data in one berkely db file to be read by a second script.
munin [http://munin.sf.net/]: Munin is a tool for graphing all sorts of information about one or more servers and displaying it in a web interface. It uses the execellent RRDTool (written by Tobi Oetiker) and is written in Perl.
If you plan to parse spamd's syslog output, some helpful notes can be found at SpamdSyslogFormat.