ArkWords is a great little app by Arktronic that features not only a great dictionary and thesaurus but an impressive, multi-language translator. In addition, it has a swanky Live Tile that updates every morning with the WOT (word of the day).
The app is superbly built with a very minimalist and fluid UI. Plus it's free, so there's little reason to not give it a shot. And now that the Live Tile has been fixed in v1.1, all seems right with the app and gets our strong recommendation.