Photo taken in SoMa, San Francisco using Instagram

Many tech companies begin in San Francisco, CA. South of Market (SoMa) is the home address and the epicenter of Web 2.0 startups. ^[1] Many of these startups go on to become hugely successful tech companies worth billions of dollars.

Here is a list of past and current tech companies headquartered in San Francisco:

• Craigslist: a centralized network of online communities featuring free online classified advertisements, with sections devoted to jobs, housing, personals, for sale, services, community, gigs, résumés, and discussion forums. 
• Dropbox: a Web-based file hosting service operated by Dropbox, Inc. that uses cloud storage to enable users to store and share files and folders with others across the Internet using file synchronization.
• Instagram:  a free photo sharing application that allows users to take a photo, apply a digital filter, then share it on a variety ofsocial networking services including Instagram's own.
• Square: an electronic payment service, provided by Square Inc.
• Twitter: an online social networking service and microblogging service that enables its users to send and read text-based posts of up to 140 characters, known as "tweets".
• vWorkApp: an online dispatch and job scheduling software for medium to large businesses.
• Yammer: an enterprise social network service that was launched in September 2008.

See a longer list of companies based in San Francisco here.

References:

1. Blank, Steve. Steve Blank > Guide. http://steveblank.com/visitors-guide/. steveblank.com. Web. March 14, 2012.