Hi, I've been intermittently lurking here since I started reading HPMOR. So now I joined and the first thing I wanted to bring up is this paper which I read about the possibility that we are living in a simulation. The abstract:

"This paper argues that at least one of the following propositions is true: (1) the human species is very likely to go extinct before reaching a “posthuman” stage; (2) any posthuman civilization is extremely unlikely to run a significant number of simulations of their evolutionary history (or variations thereof); (3) we are almost certainly living in a computer simulation. It follows that the belief that there is a significant chance that we will one day become posthumans who run ancestor-simulations is false, unless we are currently living in a simulation. A number of other consequences of this result are also discussed."

Quite simple, really, but I found it extremely interesting.

http://people.uncw.edu/guinnc/courses/Spring11/517/Simulation.pdf