The popular eMail Spamfilter written by James Farmer.
BFilter is a opensource Spam filtering HTTP proxy which
supports regular expressions and heuristics to detect ads.
X-Ray is the Swiss-Army-Knife for Mailheaders. It's a generic POP/SMTP
filter which is capable of changing/removing every part of the header or
even adding new parts to it on the fly. A NNTP client/server is also included.