According to a recent FAA Notice, the National Air Transportation Association (NATA) has set up a toll-free Hotline, funded by a grant from the FAA, for people to use in reporting suspected illegal commercial activity where an aircraft operator without an FAA Part 135 certificate is accepting compensation for transportation, in violation of both FAA and Department of Transportation regulations.
The NATA Illegal Charter Reporting Hotline is: 1-888-SKY-FLT1 (1-888-759-3581). The Hotline is staffed by an independent third-party with knowledge of the air charter industry. Reports of suspected illegal charters can be filed anonymously if desired, and the Hotline will provide all reporters with a case code for follow-up. NATA will then provide the FAA with details to initiate an investigation, and will follow up with the FAA to ensure that it is, in fact, investigating and taking appropriate action.
The original press release for the Hotline can be found here. If you have questions regarding the Hotline, you may contact Chris Monteleon at the FAA Air Transportation Division (firstname.lastname@example.org) or refer to the NATA website, http://www.nata.aero/.