If you are a Web designer and often write (X)HTML/CSS code, then the following list might be useful for you. In it, I have combined the most popular fonts on Windows, MacOS and Linux systems. The data is drawn from here and the stats are valid as of January 17th, 2009:

Windows:

Microsoft Sans Serif: 99.61%
Arial Black: 97.82%
Franklin Gothic Medium: 97.58%
Palatino Linotype: 97.54%
Verdana: 97.54%
Arial: 97.13%
Courier New: 96.96%
Comic Sans MS: 96.83%
Tahoma: 96.79%
Lucida Console: 96.76%
Impact: 96.49%
Trebuchet MS: 95.97%
Sylfaen: 95.00%
Lucida Sans Unicode: 94.34%
Georgia: 92.97%

Mac:

Monaco: 96.91%
Arial: 96.62%
Courier: 96.48%
Helvetica: 96.48%
Arial Black: 95.78%
Verdana: 94.37%
Georgia: 93.53%
Helvetica Neue: 93.07%
Trebuchet MS: 92.69%
Geneva: 92.41%
Courier New: 92.12%
Gill Sans: 91.58%
Comic Sans MS: 91.42%
Times New Roman: 90.58%
Arial Narrow: 90.44%
Apple Chancery: 90.15%
Skia: 90.15%
Lucida Grande: 90.01%
Futura: 89.73%
Hoefler Text: 88.47%

*nix:

DejaVu Sans Mono: 92.31%
Bitstream Charter: 90.77%
URW Chancery L: 89.87%
Nimbus Mono L: 89.35%
Century Schoolbook L: 89.09%

In bold above I have marked 7 font families in total. They are those fonts, which are found often on Windows and Mac (simultaneously), and these are (in mixed order):

1. Verdana (sans-serif font family)
2. Georgia (serif font family)
3. Courier New (monotype font family)
4. Arial (sans-serif font family)
5. Arial Black (sans-serif font family)
6. Trebuchet MS (sans-serif font family)
7. Comic Sans MS (sans-serif font family)

Unfortunately, none of these 7 fonts can be found among the most widespread fonts on Linux/Unix. Looks like I was wrong, when I assumed, that Verdana, for example, is one of the most popular fonts on Windows/Mac/Linux — the full stats show that Verdana is available on more than 90% of Windows/MacOS systems, but only on ~ 50% of the Linux systems…

More interesting conclusions can be made from the two available lists (short and long), for me it is enough to know that there are at least a few font families, which can be found on Windows and Mac systems at the same time.

Or, at least, your chances are quite high that one of these 7 fonts will be available… :-)