When people said .NET on Linux, we know it was about Mono.
At the beginning Mono was just a small open source project which tried to help .NET application to run on Linux, Now Mono becomes to a platform, it supports multiple language such as C#, VB, Java, Python etc.
Mono is the open source development platform based on the .NET framework, allows developers to build cross-platform applications which can be running on Windows, Linux or other platforms.
There are several components that make up Mono: C# Compiler, Mono Runtime, Base Class Library, Mono Class Library
Mono supports following platforms:
Linux Microsoft Windows Mac OS X BSD Sun Solaris Nintendo Wii Sony PlayStation 3 Apple iPhone Android x86, x86-64, IA64 PowerPC SPARC (32) ARM Alpha s390, s390x (32 and 64 bits) and more