JQuery

We have been shipping the jQuery JavaScript library with MediaWiki since the 1.16 release.

ResourceLoader
As of MediaWiki 1.17 all resources are (or should be) loaded through ResourceLoader. The default modules are stored in /resources. The jQuery file is in /resources/jquery/jquery.js. There are no static minified versions anymore as ResourceLoader takes care of this when combing and optimizing all queued files. 1.17 ships with jQuery version 1.4.2 (since r65962).

Before ResourceLoader
MediaWiki 1.16 shipped with jQuery version 1.3.2 with some patches by Trevor Parscal and Roan Kattouw to fix jQuery bug #5081 (see r60697 and r61700).

In 1.16, the jQuery file is in skins/common/jquery.js and the minified version is jquery-min.js in the same directory.

Extensions can use OutputPage's includeJQuery method, which has existed since r61431, to avoid loading minified version of jQuery core multiple times. It should be noted that includeJQuery does not support jQuery plugins and such.