Useful article on the Typekit blog from 2010 about using font events to refine font parameters when a web font is not available:
A reference to the Code Style website is mentioned:
As nimbupani pointed out in the comments, there’s a much better resource for choosing fallback fonts available at Code Style. They have data on which fonts are installed across Windows, Mac, and Linux machines. Use their very helpful font stack builder tool to see the percentage of users on each platform that each font in your fallback stack will cover (see this example). I’ve already found this resource to be extremely helpful. Check it out!
Some of the comments are useful too:
The generic serif or sans-serif should be the default fallback. The only time I would include another option to fallback to before the generic would be when something like Georgia more closely matches your intended web font. Never specify Arial and then sans-serif, or you create a situation where Mac users will get Arial instead of Helvetica (from JOE GOLIKE)