The bit that made me laugh till I thought I was going to die wasn't in a gag reel. It was the People's Choice get-out-the-vote ad. I didn't expect the end at all and that was what made it funny.
It loses it's punch once you know the ending but the socks still crack me up. The runner-up for this would have to be the outtake with Sam stretching after speed-walking. I was so impressed that Jensen didn't crack. I would have been on the floor laughing but Jensen just played along like it happens every day. It probably does.