Free, open source and cross-platform CMS based on ASP.NET Core and ExtCore framework 

A simple and lightweight SQL client desktop/terminal with cross database and platform support 

Cross-platform Single Page Applications with ASP.NET 5, Angular 2 & TypeScript 

Simple wrapper for cross-browser usage of the JavaScript Fullscreen API  Demo

A Deep Dive into the ASP.NET 5 Runtime
Go Cross-Platform with the .NET Framework 

Cross-platform development with Cordova and Ionic in Visual Studio (Part 1) 

SuperSocket is a light weight, cross platform and extensible .Net/Mono socket server application framework