eCalc is one of the best online calculator solutions I have seen yet. eCalc comes in two attractive, easy to use versions, basic and scientific. The basic version, as it's name suggests, performs the usual common functions that most students would use regularly. The scientific version performs the functions you'd expect from any scientific calculator including equation solving, complex numbers, RPN support, unit conversion, and more. And it does all this totally for FREE!
For more information, watch the eCalc video demo.