DropBox with 11 votes.
It appears simple is better when it comes to document storage and sharing.
One surprise for me was that Loopnet Mobile was not even in the top 5, even though Loopnet is so widely used, it appears their mobile app is not. I even think Loopnet's mobile app is pretty good.
While LinkedIn isn't just an app per se, their mobile app is pretty good. And is helpful when you need to look up contacts or leads which may not be already loaded on your phone.
To the 44 folks who submitted votes, thank you.