calcul8r.org is a new website I've been developing.
It's basically a calculator. It has a text box into which you type mathematical expressions in a fairly intuitive form, press enter and it gives you the answer - what could be simpler?
You can optionally sign up for an account, in which case it will save your history, allowing you to refer back to previous calculations, across multiple devices. It should even work on mobile devices, so long as they use a web browser that supports HTML5.
So go on, give it a go, and let me know what you think!