MonoDevelop is an IDE focusing on developing desktop, mobile and web applications using C# and other .NET languages. It's open source and extensible, and runs on Mac, Windows and Linux.
MonoDevelop is the IDE of the Mono Project, an open-source cross-platform implementation of the .NET standards. In addition to the Mono runtime, MonoDevelop supports .NET and C/C++; third-party plugins exist to add support for other languages and runtimes.
Bugs should be filed on the MonoDevelop bug tracker. Please ensure you are using the most recent version of MonoDevelop.
For more in-depth discussions, see the MonoDevelop mailing list and IRC channel.
MonoDevelop was the bundled IDE with the unity3d editor up until Unity version 5, in which it was replaced for visual-studio to edit C# scripts.