The introduction to the Leak Finder page on Google Code says:
Click for larger image - opens in new window
Of course, you can forget to dereference objects that you are using, and this isn't really a memory leak just an inefficiency. The new tool will help you find such problems, but it comes with a cost. You have to modify your code to make use of goog.Disposable and explicity call dispose() on an object before you free up the reference. It also works with closure by default, but you can apparently use it with other libraries.
It also only works with Chrome and isn't exactly effort-free to install.