Unity3D is a modern game engine with a deep scripting system built on top of Mono, the open source port of the .NET VM. As such it is programmable in C#, Javascript and Boo. It can generate executables for Windows, OSX, Linux(beta), Flash, IOS and Android.

Unlike most of its direct competitors, you can develop for unity on Windows, OSX or Linux (beta).

Unity excels in ease and modernness of scripting, and also has a highly scriptable editor that can be extended for integrated game specific tools. Like most game engines, its biggest limits come when you try to do things that are beyond the vision of the engine creators.

There is a limited free version and a pro version available under a number of regular and academic licenses.