(http://i41.tinypic.com/2nsx2dz.jpg)
Like the official Twitter app, this is an app that's not just notable for what it does, but by how well it works when docked to the side of the screen. As you've probably surmised, Sidebar Dictionary provides quick definitions pulled from the web, and you can browse back through previous searches and save favourite words for later use.
For $ 2.72 you can upgrade to the Pro edition, which offers built-in translations and an offline mode, alongside some other extra features.
Link (http://apps.microsoft.com/windows/en-gb/app/sidebar-dictionary/4a82fc4e-1f2e-43fb-9d2b-6ca282607d2f)