Hiding background applications

I recently was messing around and testing a few Mac OS X tweak tools and now I've go myself stuck. I've enabled an options that automatically hides all other applications when switching to another application. At first it was great, three days later it annoys me. My only problem is that I have no idea what tool I used, what preference file I touched or configuration options I switched.

Does anyone know how to turn this off?

Thanks,
Paul.