The font that deserves the very first mention is none other than Very nice Sans Serif. I have personally used this font many times in WPF and it worked wonders. The font is a bit square and thus makes the UI look a bit more futuristic. However, since the font comes in many different weights, you must manage the weights properly.
The next that deserves mention is Lightweight font. It is a very versatile font and works somewhat similar to the updated light futura. One thing to note is that it might look very thin for display in SilverLight and WPF. This is because SL and WPF tend to thin out the typefaces.
For making a Bold and noticeable display or headline, you may want to go with a very unique sans serif. APS (American Philological Society) released this font which supports Unicode well and has some additional open-type features.