Mono is a free implementation of .Net standard for Windows, GNU/Linux and some other platforms. Is compatible with .Net and has called my attention since its creation, I just needed the motivation of Miguel de Icaza to get into this.
As I'm not a .Net developer, I wanted to learn it using C#, that looks like Java, but I wanted to start with Mono because of the portability feature, applications can run in different environments, and that's a lot. Almost all distributions support or provide Mono packages, and it has beautiful IDE called MonoDevelop, that might not be as complete as Visual Studio, but I really like it.
I tried with desktop applications, that handle files and some XML processing and I have to say that is really interesting to see an application (binary) run in different environments without extra compilations.
After many years of work, Ximian team and the community have make it, it works, and it has a very promisory future, I can't wait to see what they will do next.