Some examples of good web font:
Arial: a sans serif (meaning without serifs—the little doohickeys at the ends of each letter) face that has a streamlined, more modern look, but isn't necessarily easier to read on screen because it's on the narrow side and can look very light in smaller sizes. It's also incredibly boring and looks far too much like the type used by the IRS and other tax agencies, so it can have negative subliminal connotations.
Helvetica: a sans serif face similar to Arial.
Verdana: An extremely easy-to-read sans serif face that's included with the Internet Explorer. Other fonts: Tahoma (a face almost identical to Verdana that comes with Microsoft Office), Arial, or Helvetica.
Comic Sans: This face is informal and friendly, which is great for some sites, but not professional enough for others.