Deck Simulator for Hearthstone is a tool for play testing various decks in a virtual arena. You can make any type of deck, regardless of what cards you actually own and without spending any dust for crafting. This tool is it works completely offline and has a database with the majority of HS cards, including all Naxx cards.

The app is currently available for free download on iTunes app store (iPad app).

You can see the app here:

Also I am maintaining a blog about the simulator here for anyone interested: