It depends on the specifics and complexity of what I'm trying to track. For simple things Google Analytics works pretty well. You can just do some simple searching or reports to show you the averages of user usage.

For more complex situations (often within the application itself) I've actually built some APIs which allow me to store generalized data about any page interactions and relate it back to a particular page load instance. Once I have this data I can slice and dice it to figure out exactly what users in general are doing and also allows me to drill in and follow a particular page load and what that user did specifically. I can be as complex as simulating sitting behind the user and watching them use the app. It's surprising what you sometimes learn about how a particular user or group navigates a page and how different it is from what you initially expected when you designed the app.