Comment on Drive Genius

Excellent. As mentioned, the only app (so far) that allows you to physically scan the HDD for bad sectors.
The only downside is that version 1.2 allowed you to scan the HDD from the CD when inserted (ie from Finder, insert the boot disk and run the sector scan from within OSX without having to log out). The latest version however, requires you to reboot from the CD before doing any sector scan to the HDD....a more cumbersome process (of course, this just maybe something I'm doing wrong here )

I also use DiskWarrior as well as this app (use Diskwarrior first, and this second if the problem isn't solved). I also have Techtool Pro (free with AppleCare) but don't find that as good as these other two.

Overall=something your mac shouldn't be without.