I don't generally carry or use a lock, though once in a while I kind of wish I had one. I'm kind of thinking about giving this a try:
High Strength Bicycle Lock Anti-thief 6 Joints Foldable Bike Lock - US$27.77
For $25 for a knock-off of the popular folding locks it's probably up to the minimal security needs that I have, and it looks trivial to carry.