Trelock FS300 Trigo
It's a folding lock. Fits in its own holster which mounts like a water bottle. Compact, sturdy, not too heavy and it's easy to work with. No, I wouldn't trust it overnight in NYC, but that's not what I use it for. I only carry it for emergency lockup if I break down and it's too far (or I'm unable) to walk the bike.
-Kedosto