Samparka is a PHP class that combines features of PHPMailer and Akismet. It can be used to check if submitted messages contain spam and then send the same using Gmail or any other SMTP host you have an account with.
Teak is an GUI IMAP (could also do POP3 and NNTP with some work) mail client written in php-gtk (gtk.php.net). It's still in the early stages of development but already can read/send mail via an IMAP server and local SMTP host.