I recently released Myrtille, a .NET based open source solution, and I would like to present it.
In brief, it provides a simple way to connect remote desktops (hosted on Windows Server) from a web browser.
As a web UI, it’s device and OS independant. Server side, it uses the .NET (C#) framework and the RDP protocol through an HTTP gateway.
It's particularity relies on the use of the .NET/C# framework to offer a better integration with Windows (most of the equivalent solutions, Guacamole or FreeRDP-WebConnect to name a few, are running on Linux/Apache/Java). Also, now that Microsoft has open sourced the .NET core, it makes it a real alternative to Java.
It also supports HTML4 (in addition to HTML5), file transfer and WebP compression and thus is compatible with old browsers (starting from IE6!).
It's primarily aimed at system or network administrators and developers, but everyone can use it.
I hope you'll enjoy it! feedback is welcome