Windows/Mac/Linux (Firefox): Smarter Wikipedia adds a "Related Articles" box to the mass-edited encyclopedia's left-hand menu, helping you quickly dig into and around a topic—or easily start a long spiral of Wiki-traveling. Assuming you can responsibly break away from the big grey information machine, Smarter Wikipedia is a helpful navigational tool. Beyond the "Related Articles" box, the add-on can search Wikipedia (or Wikipedia's pages via customised Google query) for any term you highlight and right-click on. The add-on is built small and light, too, as its author points out, aiming to put little drag on your open-source browser.
Smarter Wikipedia is a free add-on, works wherever Firefox does.