There's always the Wolfram Alpha app. Or if you want something offline, you could use SymPy (though obviously this isn't a touch-device app). I use Wolfram Alph

There's always the Wolfram Alpha app. Or if you want something offline, you could use SymPy (though obviously this isn't a touch-device app). I use Wolfram Alpha on my iPhone pretty much every day for simple matrix calculations, derivatives, integrals, and summations and it works very well.