Passive Spam Block List or PSBL is a real-time trap-based DNSBL operated by Red Hat kernel engineer Rik van Riel. It uses the Open Source Spamikaze in order to build and deploy an IP-based blacklist of spam sending offenders. PSBL is one of the safest DNSBL's, and has been default in Spamassassin as of version 3.3.0 released in early 2010.
PSBL receives millions of spam every day, and after taking some safety precautions, it lists the sending IP addresses. Then various organizations can download that list via rsync or query its public servers via DNS. Accidental listings can be removed at any time through the self-serve interface at PSBL.org. Otherwise, if an IP address does not send spam to PSBL within 2 weeks, then it is expired and no longer blacklisted.