Where can I find the site that finds a specific plane in the sky?

I heard of this site where you’re able to look up your plane. It’ll tell you what location it is and where to look if it’s in your area. I’m not sure what the name of it is, I just heard of it and thought it was pretty cool. My bf is flying in to town today, just thought maybe I’d try it out and see if I can spot his plane in the sky.