I find it incredibly handy to be able to just double click a word on any website and immediately be shown its definition in a simple coloured box with large type.
The only thing that *really* bothers me is if you double click on a plural word you are only told "plural form of (whatever word you double clicked)". If I'm double clicking a word, it's not because I'm confused by the presence of an 's' at the end of it, it's because I would like to see the definition of the root word. So frustrating!
Developers, please fix this!!