"That's a very nice everything you have there, it'd be a shame if something was to happen to it..."
The Creeper Voice is a saying referring to various experiences Simon goes through. It is to date the second most popular Yogscast/Minecraft quote, behind only "Diggy Diggy Hole", as some people discovered the Yogscast or Minecraft itself via this saying. It can vary, usually referring to 'everything' or something specific, like a house or structure. e.g Thats a very nice Yogcraft you have there, It'd be a shame if something were to happen to it. He often says this with a lisp.
Simon says this in the "Creeper Voice", as if the creeper were talking itself. He usually says it when a creeper is no longer an imminent threat, such as when it's trapped in a hole or glitching into the ceiling. He also sometimes says it when they are stuck behind trees or in jail.
Monty Python's Flying Circus's Mafia sketch had Michael Palin and Terry Jones as two mafia mobsters who at first say "That's a nice army base you've got there, colonel. It would be a shame if something were to...happen to it..." and then repeatedly say "X, you say? It would be a shame if something were to happen to them!".
Simon first started doing the voice in his and Lewis' World of Warcraft: Cataclysm Beta series. One of the earliest occurrences of Simon using this voice is in the YoGPoD, in which Lewis recounts the time he introduced Simon to his uncle. Simon put the voice on when talking to Lewis' uncle, which caused Lewis to become rather unnerved, saying "He doesn't usually talk like that. Do your normal voice, Simon!"
Regardless of the phrase's origins, "That's a very nice everything you have there" is now one of the most famous phrases ever to be uttered by a Youtube personality/team.