Some days I’m all about saving the extra mouseclicks. Looking up information with Firefox is pretty simple; most of the time just typing the term you’re interested in to the awesome bar gets you right to an applicable site. But Firefox add-on WikiLook offers an even easier way to fulfill one common need: hold down Shift, move your mouse over a word, and see the Wiktionary definition in a floating frame.
The definitions are hyperlinked, and there’s a link to jump to the term in Wikipedia as well. This is one of those add-ons that you probably won’t use constantly, but once you get the gesture into your memory, it’s surprisingly hard to live without when you switch computers. To me, that’s a good indication that it’s a keeper.