We covered GV Mobile last week, which interfaces nicely with the Google Voice app from your iPhone, but VoiceCentral actually does the same thing slightly better.
Both apps are essentially the same. They let you make calls and send SMS out from your Google Voice number to any other number in your contact list. You can check your voicemail and see your history, plus control which phones ring when someone calls into your Google Voice number (useful for using your landline to answer calls when you're at home). In general, they're the same.
But where VoiceCentral has GV Mobile beat is execution. The app may not be pretty (a lot of the buttons and icons look mis-matched, colorwise), but it rarely crashes, whereas GV Mobile crashes somewhat often when performing certain tasks. Also, VoiceCentral correctly matched up the history with my contact list.
They're both $US3, but if we had to recommend one, we'd recommend VoiceCentral, despite it being somewhat ugly to look at. But GV Mobile also has a free version, if you only want to make calls. [VoiceCentral]