(1) Payroll Service Providers (PSP)
(2) Reporting Agent (RA)
(3) Professional Employer Organization (PEO)
Update as of 3/10/14: Last Friday I had an IRS employment tax specialist in the office. Yes, she is auditing one of our clients (sigh). We spoke about the San Antonio PEO fraud and whether the IRS would accept 3401 status for the underlying employers.
She believes the IRS will act against the underlying employers and not even pause for any 3401 argument. Her observation: "it is a risk you take when you hire a PEO."