I was thinking about this before some more, and another one was Super Smash Brothers, the original game on the N64.
I literally had seen nothing of it that interested me and wasn't even really thinking about it's release, but a friend picked it up on the first day it came out and we got together to play it. I've never been much into fighters, but hey, we usually got a good run off the Nintendo mascots in games (we even like Mario Golf for a while, mostly for the taunts to be annoying). But ultimately, I was so pleasantly surprised by Super Smash Brothers, we logged literally hundreds of hours with each character on just his cartridge, and most of us wound up purchasing the game for ourselves as well.
Just didn't see that one coming at all. Most games I will really like I have an eye on months or sometimes years in advance.