Great Add-On Rated 4 out of 5 stars
I have used this for quite a while now and when I am not on my Firefox, I forget that it isn't installed on every computer/browser and I get sad. :(
I use it daily and it saves a lot of time and needless clicking. If you ever go on a Wikipedia journey (you know, you look up 1 thing and end up looking up a word in that article which in turn links you to another), it's so nice to just hover and get a quick detail or definition.
The only gripe I have, and the reason I took off a star, is a lack of ability to customize the search. Specifically I wish that I could tell it to use Merriam Webster in lieu of Wiktionary. In its defense, it is called WikiLook and this is a somewhat minor complaint.
Overall, I love this add-on and recommend it.
WikiLook was created with sole purpose of promoting Wiktionary, a MUCH better dictionary them M-W or American Heritage. Wikipedia was added to gain some extra popularity and to compete with Answers add-on, again with a purpose of raising general public awareness of Wiktionary existence. Supporting any other dictionaries or non free encyclopedias definitely out of scope of the project. But source code is available and nothing prevents forks from popping up.