Here's a mechanical invention that could improve your quality of life: the Turner Keyless Locking Deadbolt, which can be locked from the outside without a key. Turn a small collar around the lock on the outside of the door, and you can quickly set the deadbolt without fumbling around in your pocket for your keys.
It's a great idea, but you'd better make sure you have those keys in your pocket before you turn that lock. There might be one small catch, though.
Oftentimes the key for the deadbolt is the same one as the lock on the doorknob, so changing the existing lock over to this $45 deadbolt would then require a separate key, eliminating some of that convenience. Maybe there would be a way to match up the keys, so the same one could be used on both locks.
That said, all deadbolts should be made this way.
Turner Keyless Locking Deadbolt [The Uber Review]