None of the major software companies based in the San Francisco Bay Area (including Silicon Valley) appear to offer tours to the general public; I looked up Google, Facebook, Apple, Oracle, Hewlett-Packard, eBay, Symantec, Intuit, and Salesforce.com.
The larger firms like Google and Apple have campuses, parts of which are open to the public; however, you generally would not be allowed inside the buildings except as part of a prearranged visit such as a job interview or vendor presentation. Organized tours are only offered in a few circumstances; Oracle offers them to prospective employees, for example, but HP has no such thing at all. The Silicon Valley Guide will tell you where you can find things if you just want to take a picture next to a sign, or next to Google's lawn sculptures
As someone who has worked in software for some years, there is not much to see inside the buildings. Yes, the big firms have some impressive breakrooms, but the buildings, ultimately, are offices, not playgrounds. And for that matter, the buildings are not much to see on the outside, either— utilitarian, modern and postmodern office blocks surrounded by lawns, shrubs, and parking lots.
The most-recommended "geek" attraction is the Computer History Museum in Sunnyvale (though I myself have never been); some others are listed in Computer geek tourist attractions in the San Francisco Bay Area & Silicon Valley? There is also the Intel Museum in Santa Clara, and Apple has a store on its campus, if those companies hold any particular interest.
