Here's a link to a silverlight video player with all the basic features like progress/seek slider, full screen toggle, play/pause/resume, etc. The player is meant to be relatively simple while still sporting the basic features you'd expect in a video player. Thanks to Nick Kramer (project manager at Microsoft) for creating this player.

A few notes:
- This code is targeted at Silverlight 3 Beta. For example, it uses CacheMode property to improve performance of stretching video.
- All the code is in MainPage.xaml & MainPage.xaml.cs, the other files are pure VS defaults
- Tim Heuer started 2 versions of the SL2VideoPlayer on CodePlex (http://sl2videoplayer.codeplex.com) -- one is *very* full featured (captions, markers, etc.) -- while there also is a fork called 'min' which does exactly what this one does here.
- Known limitations -- no adaptive streaming, not common to all sites.  Minimal keyboard support (you can actually tab around but there's no focus rects), minimal accessibility support.