Microsoft Internet Explorer 'Core' font set
This font set is installed along with Internet Explorer. It differs very slighly depending on the platform – Mac or Windows – but you can count on the following fonts being present on well over 90% of all computers connected to the Web. You can view the full set here.
These fonts used to be available as a free, downloadable set but this is no longer so. This means that Linux users may no longer have them.