For a good long time we developers have been using Arial as the default font that we've used for all of our san-serifed needs, most of us too young to know why, or where it came from.
Arial is a hybrid between Helvetica and Grotesque, which mimics the same weight and dimensions of Helvetica, but using many of Grotesque's "parts" (for lack of a better word).
Arial (and all of its copycat brothers) came from Helvetica having been built into the firmware of printers, and needing something that could be "swapped out" as an equivalent. Arial is not alone in its copycatting of Helvetica, many others did it so they wouldn't have to pay fees to license Helvetica. One of the benefactors of Arial was a little company known as Microsoft.
Bundled with Windows and every version of Microsoft Office (including those for Mac), Arial soon became the font to use, even though it's arguably not as nice as Helvetica. Even Yahoo! with their YUI 2.4.1 uses Arial preferentially over Helvetica in their fonts.css file.
I argue that Helvetica is better than Arial, and Arial is simply a "knocked off" version of Hevletica, and not an improvement. Therefore, YUI (and your stylesheets) should reflect this accordingly, as such: font-family: helvetica,arial,clean,sans-serif;
That having been said, there are fonts that have theoretically improved over basic Helvetica and Arial for on-screen readability. Verdana, Tahoma, and Calibri, for example, could be argued to be improved and designed specifically for use on monitors and other displays (rather than the printed page, as was the media that Helvetica and Arial were designed).
Also published on JoeTheWebGuy.net.