From August 27th, 2013 to September 27th, 2018, 100% user give 5-star rating. for XKCD Hovertext chrome extension.
Works fine. Should have support for Explain xkcd though.
Does exactly what it says! Very customizeable, the only suggestion I may have is for the ability to change the alt-text font. 5 stars because I don't know if that's possible! I don't normally use exclamation points, but periods just don't seem to convey the enthusiasm of this review!