Wikipedia Companion add-on for Chrome is a mini wiki browser
Wikipedia Companion is a small, slick Chrome add-on that provides quick access to Wikipedia via the toolbar. You click the button and immediately get a mini browser, which you can use to quickly look up facts (or "facts," as is often the case with Wikipedia) without leaving the current page.
The mini browser has its own "back" and "forward" buttons, and it uses the cache so that going back is super-fast. There's a nice loading screen while you wait for the page to come up. I know it's a weird feature for me to mention, but it actually makes the whole add-on feel more polished. It's like you're using an application rather than a clunky mini browser window.
You can easily switch from "mini-mode" and open the page in a tab of its own. It also supports a secondary language, but for some interesting reason (politics?), Hebrew wasn't on the list. I could see just about any other language that I could think of listed there.
The one feature that's missing is having a way to quickly look up the current selection. The add-on doesn't do anything with the right-click menu, which is a shame.