If you're using your bike just to go meet someone at the coffee shop, it's a hassle to have to bring along a backpack, just to carry your lock. You could get a lock-mounting bracket installed on your frame, but the Seatylock presents an interesting alternative – as its name suggests, it's a bike seat that can be removed to serve as a lock.
The actual locking part of the device folds up underneath the saddle sort of like an Abus lock, when not in use.
When you park, the flip of a quick-release lever causes the Seatylock to disengage from an included aluminum adapter that's mounted on the existing seatpost. The plastic-coated hardened steel lock is then folded out, run around the frame and an adjacent immovable object, then secured using a key.
As an added benefit, because the saddle is part of the lock, wandering parts-thieves won't be able to steal it as easily.
Currently the subject of a Kickstarter campaign, the Seatylock should be available in two saddle versions – a streamlined Trekking model, and a fatter-but-cushier Comfort model. Slightly the lighter of two, the Trekking (including the lock) tips the scales at 1.3 kg (2.9 lb). If users wish, however, they can also swap in a third-party saddle.
A pledge of US$85 will get you a Seatylock in either style, when and if they reach production. The planned retail price is $129. You can see a demo of the device in the pitch video below.
For another take on the seat/lock combo concept, you can also check out the InterLock. A successful Kickstarter product that's now in production, it's comprised of a seatpost that houses a cable lock inside.