The DumpText Plugin
DumpText is a demo plugin, but is in itself quite useful; it'll dump the "decoded, stripped" rendering of the text, as used by body rules, to STDERR. This is pretty handy for rule developers (and the curious).
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You can also scan a massive selection of mail using either mass-check's-c switch, or with the mailbox-scanning support added to the spamassassin script in SpamAssassin 3.0.0.