It wasn't just a GUI, it was even a browser extension.

I'm surprised there isn't a popular point-and-click Windows GUI for ARP spoofing yet. Something like driftnet but all sorts of data, and with automatic spoofing done.

There is, and I've used it yeaaaars ago. It's called Cain & Abel and you can find it at http://www.oxid.it/cain.html

It has a bunch more features, but with a few clicks you can arp spoof your entire network and start logging passwords.

I've used Cain & Abel before long, long ago. It's pretty close to what I'm talking about, but I was thinking more of something revamped to be pretty and user friendly even to someone who has no idea what ARP is or what "spoof" means.

And whenever you run it, it will be automatically deleted by your anti-virus. As a budding security researcher years ago, that was super irritating.