By way of Bob Bly, I read a report from 2001 about the best fonts for website, or HTML, readability. Though the study is a few years old, the data and findings still hold true and are important to consider when you are creating web pages and HTML newsletters.
The study, by Ralph Wilson, on the readability of different fonts, found that readers prefer Ariel over Verdana and 12 pt over 10 pt.
Most print documents use a serif font like Times Roman, but reading a computer screen is much different and it seems that san serif fonts, like Ariel and Verdana, are more readable. Wilson has some interesting stats that support his findings.
I will occassionally use a serif font like Georgia or Times Roman for headlines in order to really make them stand out from the body text (example here).