ChemCrafter
ChemCrafter, from the Science History Institute (formerly the Chemical Heritage Foundation), is a free iPad app that mimics a classic chemistry set. It is set up as a game, with three sections: reactions with water, reactions with acid, and salts. The app shows the progress of the reaction (smoke, color change, etc.) when two elements are mixed in a reaction vessel, and also gives the change in enthalpy of the reaction.