US-centric: Next time you miss a call from an unknown number, check if it's a telemarketer on the CallerComplaints web site. CallerComplaints maintains a user-submitted database of telemarketer phone numbers and even rounds up the worst offenders. Registering with the Do Not Call list is your best defence against unsolicited calls, but for those telemarketers still pushing through, CallerComplaints could come in handy.
AU - in Australia the relevant bodies are the Telecommunications Industry Ombudsman which oversees telcos/ISPs and the Do Not Call Register
CallerComplaints