The only current software I wrote that some people actually use is called RA-Plugins. It's a series of proggies you plug in your SMTP server's conversation, and do diverse things with it, like rejecting messages that fail certain criteria, checking the status of the recipient's account, whatever.
Injecting this stuff in the middle of SMTP is good because it means you will reject the messages before they get into your server. But... I am running out of ideas, so... have any you can spare? :-)
You can see the current plugin list here and the only ideas I have left are:
A plugin that calls back to the sender's email server and tries to email him, ala milter-sender to catch forged senders.
A plugin to autowhitelist in spamassassin those addresses to which you send mail.
A plugin to keep an account of how many connections you hold to each IP, and limit them. (Not currently possible)
If you understand what I wrote, and have any ideas... feel free to post them as comments and/or email me with them!