For sans-serif fonts, Helvetica is your best bet for Linux machines (as well as Macs). As a fail-safe, be sure to also specify a generic sans-serif fallback option.
For serif fonts, Times will be the default on most installations.
font-family: Helvetica, sans-serif;
Generally, to handle all three major operating system platforms, I use the following:
font-family: Times, serif;
font-family: Tahoma, "Lucida Grande", Lucida, Verdana, Arial, Helvetica, sans-serif;
font-family: Georgia, "Times New Roman", Times, serif;