Blizzard has a userscript for displaying tooltips when linking to game element descriptions on their webpage. This user script adds this functionality to Gaming Stack Exchange by incorporating this script when browsing the site.
For instance, here's a screenshot from this question:
License and Info
Public domain, version 1.0.
Installation
Credits
Shamelessly copied from (the also public-domain) http://userscripts.org/scripts/show/132780.