EveryCircuit, and it runs on Chrome browsers, IOS devices (iPad, iPhone), and droids. The circuit is a simple relay driver with a diode snubber, copying something I have in real life – which let me look at the actual circuit with an oscilloscope, and compare it to the simulation. They were remarkably close, even with the component's values at the default setting.
I go looking for simulators every once in a while, hoping to find something a bit more interactive and usable than the bog-standard SPICE simulators. Those feel to me like I'm using hand tools to build a house – I wanted a circular saw, dammit! I've found some interactive simulators before, including (if memory serves) an earlier version of EveryCircuit. They were so simple as to be nearly useless. EveryCircuit is not. It is not as complete or as sophisticated (by far) as a decent SPICE implementation – but it is complete enough to serve for the vast majority of my purposes. It works remarkably well on the iPad, and despite the small screen of an iPhone it's passable there as well.
There's a very active online community with vast quantities of user-contributed circuits. I did a search for “zero crossing” and got 16 results. The quality varied from pathetic to quite good.
It's free to try (even on IOS or Android), but the trial comes with some severe limitations. To buy it is a one-time purchase of $14.99, which is the least expensive commercial simulator I've ever run across. It's now part of my electronics toolkit. I'm sure I'll still bump up against its limitations now and then, and for all I know they'll go out of business some day – so I'll still keep a “real” SPICE implementation lying about. But for the common little jobs I need to do, this will do just fine...