Craigwatch was a free service that allowed you to constantly monitor craigslist and receive instant email notifications when something you were watching was posted.

Craigwatch incorporated PHP-CLI, cron jobs, extensive PHP DOM manipulation and sent about 160,000 emails per month.

Nginx

nginx is a lightweight, high-performance Web server/reverse proxy and e-mail (IMAP/POP3) proxy, licensed under a BSD-like license. It runs on UNIX, GNU/Linux, BSD variants, Mac OS X, Solaris, and Microsoft Windows. Nginx powers 20-26% of all websites commonly surveyed, including most of the high-traffic sites.