I know it's not strictly an app as such and it's not what you asked for, but it's still worthwhile none the less.
Things like Plex, or Parallels probably, but don't really spend a lot of time on my Mac, and most if not all apps are generally available on both platforms.
Caffeine (stops the sleep mode), or quicksilver (launch bar) also.
Was just having a look and there are a couple of others to try that look decent:
Screenflow or Screenio (screen recorder)
AppCleaner
DockLibrary (provides some dock options)
NeoOffice (Free)
Vuze (Download movies)
AppFresh (updater)
Transmission