Search engine optimization, search term tools?

[Cooon F]

Hey guys, I made a few websites back in high school. I remember a website that allowed users to look up a few subjects, and it would return statistics on how many people search up similar or related terms. This was a great way to know what kind of words we're best to put in website and keywords/metatag stuff. I found this worked really well. I can't seem to find the service or any similar services... does anyone know where I can find tools like this?