Link: Mathics

Login: Offered but not required to use the site.

Enter queries and submit them by pressing Shift + Return. See the gallery for some neat examples or the documentation for a full list of supported functions. Mathics uses MathJax to display beautiful math. Visit mathics.org for further information about the project.
Example plot:



Plotted expression:

Plot[{Sin[x], Cos[x], Tan[x]}, {x, -3Pi, 3Pi}]

Found on the Gallery page [link at the bottom of the first loaded page]