Typefaces are fascinating but easy to misuse. I like the simplicity of Helvetica and Verdana and have used them almost exclusively in my CD projects. Lately I’ve started using Lucida Grande because it it a bit easier to read on a monitor. For print I use Helvetica in headers, price lists, and order forms. I usually use Adobe Garamond or Times New Roman for block text and captions.
Most web sites read better with Sans Serif fonts so I usually use these lines in the CSS.
font-family: Tahoma, "Lucida Grande", Helvetica, sans-serif;
For some sites, I’ll do the headers in sans and the body text in serifs. I think it works OK on this site.
font-family: Georgia, Times, "Times New Roman", serif;
Georgia and Tahoma are fonts that are present on most computers, so the text layout will be similar across platforms. I prefer Lucid Grande to Tahoma—it’s a bit thinner—so I usually switch positions after I’m done with the layout. Mac users get a slightly cleaner look.
If you want to learn more about which fonts designers like, this post is a good start. He makes a good point that most free fonts are crap. There are some that are well designed and can add a bit of punch to your work if used well. Vitaly Friedman’s list is a good place to start. I’m partial to Delicious and Fontin.