Hah! This was one of the first things that came to my mind upon seeing the thread title, too.For me it would be the music of Pink Floyd.
Ocarina of Time did that for me. Once you finally get out of the cramped Kokiri Forest and get out onto Hyrule Field, it just seems so vast.The first time I started playing EverQuest, and just how huge the world was and everything you could do in it was amazing to me. No other MMO has captured me quite like that one did
"Even Good Omens?"The whole of Terry Pratchett's body of works. Reading them the first time was such a joy. I would love to be able to recapture that experience.