To munger is to use a website for other than its intended use. One common munger is to look a word up on Amazon when you are at the library. Another common munger is to type a word into Google to see how to spell it correctly. These are examples of mungers because Amazon was not built to help you check books out of a library and Google wasn't built as a spell checker, but both websites can be used for those purposes.
Snooping on your neighbors on Zillow?! That's such a munger.