Bump Hammer

Bump keys are standard key blanks cut to allow for bypassing locks using the bump method. Simply insert the key almost all of the way into the lock, then tap it with the bump hammer while applying torque to the key with your non-dominant hand. When done correctly, the key will "bump" all of the cylinder pins upward simultaneously, allowing you to quickly turn the key and unlock the cylinder before they reset. 

It's even simpler than it sounds and with a little bit of practice you can open many standard deadbolts and cylinder locks in a few seconds. For ease of use you can slip a rubber O-ring on each key, which acts as a shock absorber and assists the key in returning to its original position if you need repeated attempts to open the lock. It also reduces the noise associated with tapping the key, and prevents telltale scratches to the surface around the keyway.     

If you're worried about leaving a house key outside where it can be stolen and used against you, might consider leaving a bump key in one place and the bump hammer in another. Only with both in hand can you bump the lock and get back into your own home. It's also a great set to keep if you oversee a large facility but don't have a set of master keys.