An old design school buddy of mine asked me how many fonts I had the other day. I snickered. Enough to get by, but not enough to be considered a worthy graphic designer. Truth is, my world revolves around Verdana. It is the most optimized font for MSFT apps and a very good font for web site readability. When your life is web, you live by Verdana (at least for non Mac heads).
Jensen has posted about the new MSFT font, Segoe UI, which takes advantage of ClearType. It is an interesting read. I don't care much for the name though, too much interpretation for pronunciation.