Albeit a simplistic SPICE simulator engine, there are a couple of web-enabled simulators which show animation and DC operating points for their circuits. An example of this is EveryCircuit. Again, this is not intended for heavy simulation like a Mentor product, but for a quick DC analysis of a particular aspect of a circuit, it's extremely useful.
Their UI is also suited to tablet and mobile devices which work fine on the web interface (I often prefer using a tablet or touch screen). The lack of sub-circuits and custom models prevent it being from much more than an educational tool or quick check. However their parameter entry is actually pretty good without requiring a keyboard. It also works really well as a sketch-pad for circuit ideas.