Get rid of spam at the source. Use this system to notify the correct network administrator of email abuse. Or sign up, and have your email pre-filtered to prevent spam.

WebAgent

WebAgent will check your favorite web pages for updates and sound an alarm when they change. WebAgent is a Java Applet which uses a proxy on this server to contact other network hosts - normally prohibited for security reasons.

Send Secure (Perl source code)

Send Secure is a perl script which, when combined with PGP and an SSL webserver, can provide end-to-end encrypted message sending via a web-form.

It's rather basic, and requires tailoring for your application. If you generalize it, please submit your changes.

Julianster music sharing system (Live demo with perl source code)

Julianster is a script to play mp3s from a webserver. Now includes an HTML5 in-browser music player.

Salter - Flak for your spamflow (Perl source code)

Salter is a basic single-threaded SMTP proxy which talks on a configurable source and destination IP and port. The twist is that it alters the data it passes. In the "MAIL FROM" (return path) and headers, it converts your email address (or just a placeholder address - you@example.com) into a random code (a new, salted address under your domain). For instance, you could configure your email software to send mail as you@example.com and salter will convert your emails so that all your recipients see addresses that look like: 9Nb0mZrufpHBwkwz@salty.you.example.com. You of course still have to manage the mail that comes to 'salty.you.example.com'. You must be able to disable email addresses selectively on your domain. It is suggested that you use a sub-domain for the salty area, lest you permanently poison your primary domain.

Filtermail - Filter IMAP folders

A simple imap mail filtering utility. Used in conjunction with salter, it can filter out misdirected bounces.

Mail Manager - Add-on for *nix/sendmail systems to limit mail flow from bad actors. No sendmail configuration or code changes required. This script reads a sendmail log file in real time and uses command-line firewall configuration tools to block the hosts who exceed customizeable rate-limits or cause certain errors.

HTTP Manager - Mail manager's sister. This script is a variant tailored to finding denial of service attacks and other bad behavior automatically and then filtering them. I guess the mis-used term for this is "intrusion detection." There really isn't any intrusion, these are just annoyance attacks.