Mac: If you frequently visit Wikipedia with your questions and want the encyclopedic resource to be one click away, Qwiki for Mac brings the site to your menu bar without ever having to launch your browser. The simple app adds a Wikipedia search field directly to your top menu bar, so you no longer have to stop what you're doing to launch the site every time an trivia question is itching the back of your mind. You can also copy the link to the article you're reading, open it in your browser or send it off in an email or tweet or anywhere else you'd might like. It's not free, but it's just three buck.
Qwiki ($2.99) [App Store]