A number of San Francisco city directories are now available online through the Internet Archive website. This means you can browse through directories from as early as 1850 up to 1953 from anywhere with Internet access.
As you may know, these directories can be a source of great genealogical information such as helping establish residency dates, residential property status, and providing spouse and employment/profession information.
Hint: To find the directories, use the "browse by title" list and select "T" (for "The San Fran..." roughly for the years 1850-1879), "C" (for Crocker-Langley 1891-1929), or "P" (for Polk's 1931-1953).
Information about the Internet Archive and its mission is available here: http://www.archive.org/about/about.php