That would be https://www.circuitlab.com/ -- I built it :)

Edit: if anyone wants, reply with your CircuitLab username and I'll give you a free membership upgrade when I get home this evening.

Been using iCircuit for iOS for a very long time, which is a C# port of falstad's engine. Only recently have I tried the demo of circuitlab; It would be much appreciated to give it a more comprehensive try... Username: up-n-atom

Thanks!

Username: yuhe

Thanks, that's very generous! I registered a while ago and I'd definitely pay for something like this if I was using it professionally, but I'm barely a hobbyist...

Username: snowdem0n

Very generous of you. I've loved using it to answer stackexchange answers but don't have enough use myself to justify the membership.

Just signed up, username: DavidGould.

I'm just getting started, so not sure what tools I'll end up using most, but it great to get to try this.

Thanks for building that. Awesome piece of software. Helped me a lot when I was dabbling in hardware coming from software.

Thank you, I'm just at the stage where I'm learning schematics and this looks like a fantastic tool!

Username: fluxsauce

I've been starting out with electronics tinkering, and this looks awesome! My username is jmgao.

Username is hn50

Many thanks.

I remember running Spice on an 8086... The day I bought an 8087 coprocessor...it was SOOOOO much faster!

Cool, just signed up as kstenerud. Never knew something like this existed!

Cool - my username is "seandss" - much appreciated, thank you!

If that's also true for brand-new users, the username is the same!

Thanks! I'm quentinv

username: domsl

Hey I just signed up, it looks pretty cool. I still remember struggling with spice at school (almost 10 years ago) – I think this makes easier access to electronic simultion :)

Just signed up, username ulmas.

Thanks so much, Mike, so generous of you!

Username: tpdc

Thank you for the offer, even if you don’t get to me.

This looks amazing, thank you! username: dujoducom

Hey that's cool. Username is efosmark.

Thanks, great site! My username is TKems

Nice! This looks really cool :)

Username: RandomException

Username: k_samson Thanks. Great job!

Many thanks: Username: braintoaster

Yes please. username: pkaye

Thanks! I'm sprawl2

Thanks username: asusta

Yes please: chromebin

Thanks! I'm wrs.

Username: illuminati1911

Thank you! :)

A big fan since 2012!

My username is magnetik

Thank you!

Thanks for the generosity:

kemalenver

Awesome - thanks!

Username: kburkitt

Username: nikolay

Thanks from kqr2!

username: iceychris Thank you very much :)

just signed up , username : [redacted]

mine would be: lonewolf72. Thanks heaps

username: bxtel

Thanks! Looking forward to using this!

if not too late, nullmeme Thanks!

ade2 if it's not too late!

Username: ivd0

Thanks!

I want!

username: blarg1

Thank you!

Bjartr

username: michaa Thanks :)

user: kpatrick Thank You

thanks, mine is rkasturi

username: Raft Thanks!

asampal

Thanks so much for the upgrade!

username: CharlesInCharge

Thank you!

username: vkhurana

Thanks man! :)

ussername: shesto

thanks man ! :)

username: rogier

Thanks!

uscpablo

Thanks!

nerfhammer

do0g

Thanks Mike!

aallbrig

Thanks Mike!

username: Raft Thanks!

If you don't mind using a desktop app, there are still many great tools for hobbyists and for teaching.

Recently I've used: Qucs: https://sourceforge.net/projects/qucs/

Circuitmod: https://sourceforge.net/projects/circuitmod

These were also in my bookmarks of tools I've explored: CEDAR Logic Simulator: https://sourceforge.net/projects/cedarlogic

Logisim: https://sourceforge.net/projects/circuit/

For teaching LogicWorks is still my favorite.

KiCAD 5 was recently released (congrats team!) and it got ngspice integration which sounds neat. Haven't yet had time to try it out, but what I've seen so far looks promising. Of course with SPICE (and really any simulator) the problem is creating/finding good models for your parts or even generic ones.