These are no- or low-cost legal agencies that have experience with HUD and SFHA cases that serve low-income tenants.

Bay Area Legal Aid (BALA)
50 Fell Street (415)982-1300 M-F, 9-12; 1-5
Legal Advice Line: (415)354-6360

Homeless Advocacy Project (HAP)
For eviction representation. Especially good for disability-related cases.
1360 Mission Street (415)575-3130 Drop in clinic Tuesday 1-4

Legal Assistance to the Elderly
995 Market Street, Suite 1400 861-4444 M-F; 9-12; 1-5

Eviction Defense Collaborative (EDC)
Go to them when/if you receive eviction papers (a summons for an "Unlawful Detainer"). They will assist you to file the proper forms but they will not represent you in court.
995 Market Street at 6th Street, 12th Floor, San Francisco, CA 94102 M-F, 9:30-11:30; 1-3

Volunteer Legal Services Program (VLSP)
This is run by the Bar Association. They will refer you to a private attorney for a consultation but it will cost a fee.
465 California, Suite 1100; (415)982-1600

Aids Legal Referral Panel
For tenants with HIV/AIDS
1663 Mission, Suite 500 701-1100 Call for appointment.

Asian Law Caucus
939 Market, Suite 201 (415)896-1701 M-F 9-5

Tenderloin Housing Clinic
For SRO tenants and residents of the Tenderloin and SOMA.
126 Hyde Street (415)771-9850 M-F: 9-5

La Raza Centro Legal, Inc.
Spanish speaking attorneys.
474 Valencia (415)575-3500 M-F: 10-12; 1-4

New College Small Claims Clinic
If you think you have a small claims case.
50 Fell Street (415)241-1390