CS6200 is awesome (currently in it myself this term). The projects alone have been an excellent exercise, mainly because I come from a web development background. The papers we are reading are very interesting. Prof. Gavrilovska does a great job explaining concepts that I found confusing in my undergrad OS course.
The lectures are free and the course can also be taken as part of OMSCS.
Fantastic course really. It teaches something new and the theory behind it all is really interesting.
Other top-rated OMSCS courses such as CS6200 GIOS are really good as well.