You need the free version of Unity located at http://unity3d.com/unity/download/. The MonoDevelop code editor is included in the Unity installation.
Your computer will need to meet the minimum requirements for Unity as specified at http://unity3d.com/unity/system-requirements.html.
Windows: XP SP2 or later; Mac OS X "Snow 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.
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