I am attempting to learn more about mobile/tablet development and, as a .NET developer, I am naturally drawn to Mono. But I am confused--I thought that Mono was free and op
Update (2016/03/31)
Mono has been re-licensed to MIT;
The Xamarin.iOS, Xamarin.Mac and Xamarin.Android SDK are going open source;
History answer
First let's clear up the Mono case :-)
Mono is free as in free speech, not free beer. You can get it for free (i.e. without paying) as it is available under open source licenses that allow this. But you could still pay for it too (e.g. people did pay for some versions of Mono in Novell's days). Also some people cannot use Mono under the free open source licenses (e.g. as a game engine where the source is not available...). In such cases a commercial license, from the copyright holders, is needed.
Mono for Android is a commercial product based on Mono and sold by Xamarin (who has the rights to license Mono). It re-use a lot of what Mono has achieved and also brings a lot more features to allow .NET developers to use their skills on the Android platforms.
Finally evaluation versions of Mono for Android are free and fully functional when used with the Android emulator (not with devices). So you can still try, learn and enjoy Mono and Mono for Android without paying a dime to Xamarin :-)
note: the same applies for MonoTouch which allows you to build .NET-based iPhone/iPad (iOS) applications from an Mac OSX computer.
Disclaimer: I work for Xamarin