For most entry doors, standard horizontal dead bolts will provide all the protection you need. But where an extra measure of security is desired -- or if lost or forgotten keys are a problem at your house -- a specialized dead-bolt lock may serve you better.
Keep forgetting (or losing) your keys? Keyless locks dispense with keys altogether -- they operate by entering a combination on a numerical keypad. And if you're ever concerned that someone may have learned your combination, you can easily reprogram a new one yourself -- a much cheaper alternative to rekeying a standard dead bolt if you lose a set of keys.
Vertical dead bolts won't win any beauty contests, but they may be just what you need for a high-security door. They make it impossible for an intruder to gain entry by forcing a pry bar between the door and the frame -- a tactic that works with many horizontal dead bolts (especially those with a bolt that throws less than an inch into the jamb). The interlocking fingers on this lock's mating parts are held together by the vertical bolt that runs through them, making the lock impossible to pry apart.
Noisy and decidedly industrial-looking, sliding bar locks mount conspicuously in the center of the door. Turning the key or inside knob drives two long dead bolts into opposite jambs, making the lock almost impossible to jimmy. Unlike other dead bolts, these locks keep the door secure even if an intruder removes the hinge pins. Consider using a sliding bar lock if you live in a high-risk location or for extra security on a rarely used cellar door.