Like most things, I'm probably the last to know about this, but it's very useful.
When I'm locally developing Django applications which use SMTP, I usually get stuck in a cycle which goes: make request, send, wait, check inbox, wait, wait, there-it-is, oh, it's wrong, I've wasted 3 minutes. Annoying.
This snippet runs a local SMTP server, which echoes all incoming mail to stdout, instantly:
sudo /usr/lib/python2.5/smtpd.py -n -c DngServer localhost:25
Note that sudo is only required to run it on the standard low port, but you can specify a different port number in settings.py if you don't have privileges.
Date: 21st September 2009.
Reading time: Less than 1 minute.