The Social Security and Medicare tax paid by the self-employed on their net earnings.
Employees split Social Security and Medicare taxes with their employer; the self-employed pay both halves, totaling 15.3% on net earnings up to the Social Security wage cap (Medicare continues above it). You can deduct half of it from income. Retirement-plan and other business deductions reduce the net earnings this tax is calculated on.
A freelancer owes self-employment tax on net profit in addition to regular income tax.