There are other (slightly) similar ideas:

1: tarpitting with qmail-smtpd (using qmail MTA):
    http://www.palomine.net/qmail/tarpit.html

2: teergrubing:
    http://www.iks-jena.de/mitarb/lutz/usenet/teergrube.en.html

3: "spam throttling content filter" for Stalker CommuniGate Pro mail server:
    http://www.cse.fau.edu/~valankar/CG_spam_filter/README

4: rate limiting by host (using exim MTA):
    http://www.exim.org/exim-html-4.10/doc/html/spec_13.html#IX981

5: "recipient limits"/"slowing down bad clients" (using postfix MTA):
    http://www.postfix.org/rate.html#recipients
    http://www.postfix.org/rate.html#slowdown


If you have additions/notes for this list, please let us know.