parp is a powerful, extensible e-mail filter with sophisticated anti-spam capabilities. It's designed as a complete replacement for procmail, is MIME-aware, and acts as a filter, daemon, or on mailboxes. It uses the Open Relay Database as part of its spam-detection heuristics, and it has a DBM-format database of trusted e-mail addresses to ensure that false positives are kept to a minimum. It can perform duplicate removal by message ID, and has many other features.
polcnv is designed to convert files between different encoding methods used for Polish texts. It can be also used to covert plain text documents in any language using supported character encoding methods. The program uses ISO-10646 UCS-4 (equivalent to Unicode UTF-32) as internal representation.
POPMail is a POP3 email checker and spam-eraser: just a way to delete spam from your POP3 Mail server based on regular expressions for any mail header (From, Subject, etc.). You can also specify a maximum email size so that spam messages are deleted before downloading them. You can disable anti-spam capabilities and use POPMail just to see the number of messages in a POP3 server, and the subjects, senders, and sizes of those messages.
The procmail and formail mail processing package can be used to create mail-servers, mailing lists, sort your incoming mail into separate folders/files (real convenient when subscribing to one or more mailing lists or for prioritising your mail), preprocess your mail, start any programs upon mail arrival (e.g. to generate different chimes on your workstation for different types of mail) or selectively forward certain incoming mail automatically to someone.