The Google Android Emulator is part of the Android SDK which can be downloaded free of charge from http://developer.android.com/

The SDK will download as a ZIP archive file. Extract the contents of the ZIP file to a location on your hard drive.
On the Mac platform, open the Terminal and browse to the Tools folder within the extracted ZIP file contents.

The first thing you need to do is to create a virtual Android device. Do this at the command prompt by entering:

./android create avd -n (NAME) -t 1

where (NAME) is the desired name you wish to assign to the virtual device.

Once the virtual machine has been created, you can invoke it with the following command:

./emulator -avd NAME

The virtual device will then be displayed: