I don't think I can beat some of the low temps listed here (except maybe riding dirt bikes on ice) but the coldest I ever got was on a ride home when I was in the Navy. I rode from Norfolk, VA to Akron OH straight thru. It was fall and when I left Norfolk it was shirt sleeve weather (yeah, I was a double squid). By the time I was a couple hours out of Norfolk it was in the 30's. By night fall a light snow was falling (not sticking, just flakes in the air). By the time I got to Akron it was about 2 AM and 29°. When I got into town I found I couldn't work the clutch very well and it was extremely difficult to take my feet off the pegs. When I pulled into my friends car port, I literally fell up against his van because I couldn't get my foot down.
It took about 6 hours of blankets and "spiced" coffee to get my temperature back to normal. It was really stupid, I was young, dumb and indestructible (at least in my mind). Now I live in San Diego and only ride on days when the temp is above 45°!