Updated March 31st, 2016:
We have announced that Visual Studio now includes Xamarin at no extra cost, including Community Edition, which is free for individual developers, open source projects, academic research, education, and small professional teams. There is no size restriction on the Community Edition and offers the same features as the Pro & Enterprise editions. Read more about the update here: https://blog.xamarin.com/xamarin-for-all/
Be sure to browse the store on how to download and get started: https://visualstudio.microsoft.com/vs/pricing/ and there is a nice FAQ section: https://visualstudio.microsoft.com/vs/support/