Recently, I was researching which fonts were the most prevelant throughout all types of servers and I came across this study performed by codestyle.org. They have been surveying system owners for several years now and tabulating the data in various ways. According to their top most chart, the following ten fonts are currently the most common:

Mac - American Typewriter - 96.00%
Mac - Monaco - 95.96%
Mac - Arial - 95.83%
Mac - Arial Black - 95.68%
Mac - Helvetica - 95.57%
PC - Arial Black - 95.52%
Mac - Courier - 95.31%
PC - Verdana - 94.37%
PC - Comic Sans MS - 94.04%
PC - Arial - 93.97%

Interestingly enough, my favorite font (Helvetica) and my least favorite font (Comic Sans) made the top 10.

Top 40 Font Survey

So much for our work location teaching me to love tahoma + calibri!