I once thought I was going to make an ARG, but I never got around to it, not to mention it'd be pretty lame since I never even though of giving it some sort of storyline/real mystery so it'd really just be a game of decoding something through whatever method. I do have a pretty large amount of ideas on hiding information as a result of this; here's a partial list since I really won't ever make an ARG:
-password locked .pea archive file
-finding a download link to a file w/ info/leads/whatever
-hiding information using alternate data streams
-strings encrypted with some algorithm on
this site
-sstv signals
-distorting sounds/audio through messing w/ equalization
-distorting sounds/audio by time compressing it then expanding it back out again (gives a pretty neat effect imo)
-
verylow quality video
-image blurred just enough to be able to be enhanced (deblured) to be useable
-text hidden through a very small difference in color on a all black/white/any other solid background
-gif frames (gif image doesn't appear to move but info is hidden in a frame)
I hope those ideas are useful in making any future ARG (or not since now anyone would know exactly what to do to recover the desired information). There are plenty more hiding in my like 50 stickynotes on my desktop that I'm too lazy to search through.