Unity - a multiplatform tool for game development, from the very beginning created to facilitate your creativity. Unity provides a comprehensive set of professional quality applications, and quite by accident, contains the most powerful engine this side of a million dollars.
System Requirements for Unity development
Windows: XP SP2 or later; Mac OS X: Intel CPU Leopard 10.6 or later. Note that Unity was not tested on server versions of Windows and OS X. Graphics card with DirectX 9 level (shader model 2.0) capabilities. Any card made since 2004 should work. Using Occlusion Culling requires GPU with Occlusion Query support (some Intel GPUs do not support that).
System Requirements for Unity iOS development In addition to the general system requirements:
Only Unity OS X version supports iOS as build platform Xcode 4.x
System Requirements for Unity Android development In addition to the general system requirements:
Android SDK and Java Development Kit (JDK) Android authored content requires devices equipped with: Android OS 2.0 or later Device powered by an ARMv7 (Cortex family) CPU GPU support for OpenGLES 2.0 is recommended
System Requirements for Unity-made Content
Windows XP or later; Mac OS X 10.5 or later. Pretty much any 3D graphics card, depending on complexity. Online games run on all browsers, including IE, Firefox, Safari, and Chrome, among others.