We do not provide demo versions, instead you can get a full version and a free trial key, or buy a developer key for it.
The trial key is free and can be used for testing purpose only on one computer and it can't be used for commercial projects. For additional computers you need additional trial keys.
The developer key doesn't have any limitations except that it can be used only by one developer.
Any .NET or Mono based programming language.
Any .NET version and Mono 2.0 or higher, including Unity 3D.