We have several highly complex ASP.NET web applications that are used internally by hundreds of users. We are trying to figure out which areas of the applications to invest in to improve functionality, but we aren't sure which screens/features are more heavily used. So, ideally, I'd like to find a way to add a layer of instrumentation to the applications that gathers metrics on which buttons are being clicked, which text boxes are being used, etc.

Are there any products / open source apps out there that will do this sort of instrumentation for ASP.NET? Obviously I could do it myself manually by going into the code and injecting logging statements everywhere but this would be a significant amount of work that will be hard to accomplish.