If you're a serious search hound who often clicks through to Wikipedia pages that Google digs up, then you'll love Googlepedia. This free Firefox add-on splits your Google page in half: On the left are your regular Web results, and on the right (where AdWords would normally appear), you're presented with a Wikipedia article based on Google's top result.
Of course, typing "Wikipedia"--followed by a subject--directly into Firefox's Location Bar is just as easy, but you don't get to scroll through Google links at the same time. And usefully, Googlepedia also lets you expand, shrink or hide the area that an article is viewed in.
By default, the add-on presents internal Wikipedia links as clickable Google searches, though you can toggle this in its preferences. You can also change the default Wikipedia language.
Articles from the open source encyclopedia appear surprisingly soon after Google's own always-speedy results. A good thing, as Firefox seems to take a slight performance hit for the second or so an article takes to load.
Note: The author has recently released an early port of the add-on for Google's Chrome browser, and mentions that Safari and Konqueror versions are planned. This link takes you to Mozilla.org, where you can download the latest version of the software.