Guess which iPad app brought in the most revenue this Christmas? It wasn�t Angry Birds or Fruit Ninja. It was Quickoffice, a productivity tool for viewing and editing Microsoft Office documents. Alan Masarek, Quickoffice�s chief executive officer, shrugs off the success. He notes that the app was regularly among the top-three highest-grossing apps throughout 2011. �It�s fun when you get to No. 1, but we�re pretty used to it,� he says. Almost two years after iPads started drawing oohs and aahs around conference room tables, demand for tablet-ready corporate software is taking off. After testing the waters in 2011, companies are expected to buy $10 billion worth of iPads this year and $16 billion in 2013, according to Forrester Research. Workers who already own tablets use them more often than their laptops to view office documents, says Forrester analyst Sarah Rotman Epps. And with big productivity software makers like Microsoft and Oracle mostly steering clear of tablets for now, a number of startups are trying to fill the void. They�re honing applications for stock traders, salesmen, spreadsheet wizards, and restaurant chains. �We�re just at the beginning of a renaissance of the post-PC productivity software business,� says Epps.