Navigation popups is a script, written in JavaScript (source), that offers easy access to article previews and several Wikipedia functions in popup windows which appear when you hover the cursor over links.

• Quick installation: Enter Special:Preferences and click "Gadgets", check the box to enable Navigation Popups, under the "Browsing" section, then click save. Follow the instructions on the page to bypass your browser's cache.
• Other installation method: add {{subst:navpop}} to your current skin's .js page.