For my desk, I use the little round "blind spot" mirrors. They're cheap, give a wide viewing angle, and can be stuck almost anywhere.

I know the original poster typically builds .NET applications, but for those times when you want to do this in Java, there are at least two good engines: http://www.mandarax.org/ is a backwards-chained engine (like Prolog), and http://www.drools.org/ is a forward-chained engine (takes action based on input).