Carrying a bike lock around with you while you ride can be a pain. Where do you keep it? But the Küat lock solves that with a rather ingenious solution.
Essentially, the Küat is a standard cable lock that lives in a water bottle-shaped container, letting you carry it easily right on your bike. For $34, it's pretty reasonably priced, too. The only downside is that cable locks are less secure than U locks, so if you live in an area in which bike thefts are an issue or you've got a particularly valuable bike, you may want to skip this for something a bit more secure.