"Ampsoft":http://www.ampsoft.net/webdesign-l/WindowsMacFonts.html explains what "browser safe fonts" are available. These are fonts that are both on Windows and the "Macintosh":http://en.wikipedia.org/wiki/List_of_fonts_in_Mac_OS_X. "Codestyle.org":http://www.codestyle.org/css/font-family/sampler-CombinedResults.shtml has a study of what fonts are available on how many computers. Also, if you love typography, check out "Typowiki":http://typophile.com/typowiki.
You can also be a little more aggressive and assume that Microsoft Office is installed and so are its "fonts":http://support.microsoft.com/kb/837463. These add some good ones like Bell MT, Franklin Gothic Book, Gill Sans MT, Century Schoolbook
There are not a large number of them that are common. Of these, the one I miss the most is for the san serif family. Namely, "Optima":http://typophile.com/node/15685? which was also designed by Zapf in 1958 and then updated to Optima nova in 2002. It is on 87.60% of all Macs but only 3.58% of all Windows machines according to "Codestyle.org":http://www.codestyle.org/css/font-family/sampler-CombinedResultsFull.shtml
For serif fonts:
Palatino or Palatino Linotype. The clone is called Book Antiqua and is available on the Mac if you've installed Microsoft Mac Office. This was designed by Hermann Zapf in the 1940s. 97.1% of all Windows machines have it. There was widespread cloning of Palatino according to "Typowiki":http://typophile.com/node/16864? and there has been a family, so first there was Palatino in the 1950s, then Palatino Linotype which used OpenType and then in 2003, Palatino nova
For sans serif fonts:
Lucida Sans Unicode or Lucida Grande. "Wikipedia":http://en.wikipedia.org/wiki/Lucida_Grande says it is a humanist sans-serif font by Charles Bigelow and Kris Holmes. It is very similar to "Lucida Sans Unicode":http://en.wikipedia.org/wiki/Lucida_Sans_Unicode also designed by Bigelow & Holmes in 1993. 93.48% of all Windows machines have it. 91.25% of Macs. It is a large font which could be an issue.
Trebuchet MS. This is in both Windows (95.29%) and Mac (92.7%). It was developed in 1996 as a machine readable sans serif face. "Trebuchet":http://en.wikipedia.org/wiki/Trebuchet_MS is the deffault font for Windows XP title bars. According to "Microsoft":http://www.microsoft.com/typography/fonts/font.aspx?FID=2&FNAME=Trebuchet%20MS it was created along with Verdana and Georgia to be screen readable. Trebuchet is supposed to be energetic (whatever that means.
Verdana. Both Mac (94.12%) and Windows. (97.14%). "Wikipedia":http://en.wikipedia.org/wiki/Verdana says it is like Frutiger and designed to be readable at small sizes. According to "Microsoft":http://www.microsoft.com/typography/fonts/font.aspx?FID=1&FNAME=Verdana it resembles Frutiger and also the typeface found in the London Underground. .
Tahoma. Both Windwos (96.09%) and Mac. "Designed":http://en.wikipedia.org/wiki/Tahoma_(typeface) in 1994 for Windows 95, it is very similar to Verdana but is narrower and with a tighter spacing. It is the default screen font on Windows 2000/XP/2003 and also Facebook. With OS X v10.5, it is not bundled with Macintoshes