Google offers an array of freely-available (open source, actually, I think, but I'm not 100% sure) fonts that are hosted on their servers that you can use.
To quote from the "introduction" dialog [0]: "Hundreds of free, open-source fonts optimized for the web".
Looking further, the "about" page [1] provides a bit more information: "All the Google web fonts are open source. That means that you are free to to use them in any way you want, independent of whether you're working on a private or commercial project."
I'm not sure what license they're using, though . . . I can't find anything but Google's standard "terms of service" license anywhere on the site.
But of course Arial is still a good default.