Google Analytics Debugger - Prints useful information to the JavaScript console by enabling the debug version of the Google Analytics Javascript.

Tag Inspector - Chrome extension that displays information about what 'tags' (e.g. Google Analytics, Custom Events, etc) are firing on the current page. Also displays javascript & data layer variables, and has a recording function to capture and replay DOM tag/event firing order.

Page Analytics by Google - Chrome extension for toggling on-page analytics, including click & goal heatmaps