This is the main serpentine belt tensioner. If you have rear A/C, see the secondary tensioner/damper assembly.

If you have had premature failures of this tensioner or the serpentine belt, you most likely have a failed alternator pulley.  The alternator pulley is a declutching pulley and has a one-way clutching mechanism to minimize belt flutter.  Belt flutter places stress on the entire belt drive system and the tensioner acts as a spring-loaded shock absorber.  If the declutching pulley has seized, it will greatly shorten the life of the belt tensioner and often causes the pulley to break off or a fracture of the pulley arm.  Replace the alternator pulley or the alternator in order to solve this problem.