There are lots of error logging providers available such as log4Net, AnLogger, etc. they are easy to use as well, but just to keep it simple, I have written my own. I call it "Activity and Error Logger", it stores the data in XML format, request by request and session by session, you can actually create the user behaviour maps if you properly call the activity log at while performing any major operations. It also takes care of the concurrency, two threads will not overwrite each other. It logs the errors on page in try catch block and also at global level, in Application_Error event in global.asax.
