You might be thinking of https://lightsats.com/
Discussion
Yeah that was it!
Lightsats was the one I was going to mention, too. But there is also now a Nostr specific way of doing something similar for npubs that don't have a lightning address. https://npub.cash/
Another one that I know of and have tried is https://acceptln.com/
Allows you to send sats to any email address and they can claim them via Lightning.