Lucida Grande and Lucida Sans are both commonly seen as the primary typeface for body text on various websites and blogs, Facebook being just one example. It’s most recognizable, though, for its use throughout the user interface of Mac OS X.
Helvetica arguably one of the most popular font used by designers. It's simplicity makes it very versatile and extremely legible. (Developed by Max Miedinger in 1957 with Eduard Hoffman, for the Haas Typefoundry.)