EverLock bike lock doubles as a seatpost
While bicycle U-locks may provide better security than their cable counterparts, the things can be a hassle to carry around in a backpack. The EverLock was created in response to this problem, as it's actually built into the bike's seatpost.
Developed by UC Berkeley students Andrew Shacker, Dennis Tan and Erica Gao, the EverLock consists of a main crossbar (which doubles as the actual seatpost) and a solid steel U-shaped shackle that slides into two holes on that crossbar's backside. A keyhole on the seatpost allows users to lock and unlock the shackle from the crossbar.
When users are riding, the EverLock simply serves as a seatpost. If the saddle needs to be lowered farther than the shackle will allow, that shackle can be removed and carried separately (although that would kind of defeat the purpose of the whole thing).
Once it's time to lock the bike up, an included quick-release lever is used to release the EverLock from the bike frame's seat tube. It's then simply pulled out and used like a regular U-lock, albeit one with a saddle on one end. This keeps thieves from stealing the seatpost and saddle, as the seatpost actually is the lock.
As an added bonus, plans call for the device to be equipped with a GPS module, which is powered by a battery that ought to be good for three years. If the bike gets stolen, this should allow users to track its whereabouts online.
Should you be interested, the EverLock is currently the subject of a Kickstarter campaign. A pledge of US$150 will get you one, when and if they reach production. And if you're OK with using a cable lock, the previously-Kickstarted InterLock seatpost has one of those built in.