I was google searching for sandbox games and found Minecraft while it was still in classic.
I didn't get very interested in it, but way later at the ROBLOX forums people started talking about it. I watched a couple let's plays of the now released alpha version and eventually bought it.
I was in a tech class at my school and one of the biggest nerds(besides the teacher) i'd ever met had found it and was playing one day in class. within the week all 15 of us were playing and had our less-geeky friends playing it too
word of mouth and indy game sites led to small dedicated fanbase that exploded onto youtube after reaching a sufficient size(around when they released Alpha) the originality and many features that began to be added turned a simple game about breaking dirt into an addicting time-sink of amazing proportions. that's all I got.