Registered developer accounts for Unity Technologies and its Unity 3D games engine have surpassed half a million, the company announced Tuesday -- more than double where they were a year ago.Of the 500K accounts, 150,000 are described by Unity as active monthly users, who use the company's products a combined one million hours every month.In addition, installations of Unity's browser plug-in have surpassed 60 million, the company said."The combination of Unity's powerful yet approachable technology has resulted in it being the number one game engine used for both traditional and casual games," said David Helgason, CEO of Unity Technologies. "It's exciting to see how our users build on top of Unity to create mind blowing innovation of their own."Registration has been on an intense upward surge over the past two years. Between its launch in 2005 and November of 2009, the company attracted just 13,000 licensees. That number leapt to 250,000 in November , and has doubled since.The Unity game engine was named the number one game engine by readers of Gamasutra sister publication Game Developer magazine in its May issue, which is available now as a digital download