What is the best free hard drive diagnostics app for Mac OS X?
About the only "free" tool that's worth anything is smartmontools, IMHO. 

smartmontools is a free SMART monitoring application that comes from the Unix, BSD, and Linux world (OS X is based on BSD). This tool can provide extensive SMART monitoring but it's command line which a lot of people don't like. Everything else costs money - I suppose some people have actual costs, like food, water, heat, mortgage, etc. etc.

ZVH, over on MacRumors, has written a list of drive testing tools but beware most of these cost money:


  Mac hard drive test software - creating the definitive list
  
  
  Disk Utility, diskutil (FREE, comes with OS)
  Smartmon Tools (FREE, Open Source)
  SMARTReporter ($4.95 for commercial version, but an older version is FREE)
  Scannerz Lite ($21.95)
  Scannerz with Phoenix and FSE-Lite ($39.95)
  Disk Tools Pro ($79.99)
  Disk Warrior ($99.00)
  Drive Genius ($99.00)
  TechTool Pro ($99.99)
  


That's actually a really good report. Interesting that nearly no one commented on it. In any case, I think you get what you pay for. Tools like Disk Warrior and Scannerz are known to do very limited things but do an extraordinarily good job of doing it, while other "Swiss Army Knife" tools apparently do a decent job, but not really all that thorough. The author only identifies a few applications as "free."

Although I find SMART status useful, I wouldn't base my life around SMART reports and would recommend that people do web searches about the reliability of SMART testing before using it as a defacto standard.