There really isn't a good built in monitoring option for IIS. I would suggest writing some basic scripts to monitor availability. If you're not into creating your own, if you google http monitoring scripts you'll find a ton of different options, many of which are free. Or if you need a more advanced solution, Microsoft's System Center Operations Manager is a great solution to monitor the health of your entire infrastructure:
http://www.microsoft.com/systemcenter/en/us/operations-manager.aspx