Robby Ingebretsen has published his favorite 10 open source fonts that can be embedded in both WPF and Silverlight.
Titilium - Very nice sans serif that works well for display. Comes in many weights. I’ve actually used this font a lot in WPF for display and had good luck with it. It’s a little square so it leaves the UI looking a little futuristic if you don’t manage the weights properly.