NOTE: I DID NOT MAKE THIS. I JUST FELT IT WAS GOOD TO SHOW.
Well I found a great series on youtube that goes over the ever referenced and popular IRIS ARG. Called "Perceiving the Dimensions of IRIS". It goes through IRIS in a easy to understand way. And well I recommend giving it a watch even if you know the ARG, its a good recap. You might even learn something. Plus if you don't know much about IRIS then this is a perfect thing to go through.
IRIS was a Halo 3 ARG that went over one could say the background of the Forerunner Flood war. Pretty much if your interested in ARG's or The forerunners in general watch this.