Motor drivers and kickback diodes
From the comments @ sparkfun (Specifically superbrad):


  The L293D has internal flyback diodes, which catch what would
  otherwise be high voltage inductive noise (sometimes even in the
  kilovolts). With the SN754410, you must use external flyback diodes in
  your H-Bridge.
  
  Even though you might feel like you can get away with it, it's a VERY
  bad idea to use the SN754410 without a flyback diode. It's not just
  the driver you have to worry about, it's your entire circuit (MCU,
  accelerometers, expensive XBee, etc.). Eventually, your luck will run
  out, even if you don't turn it on and off frequently.