The Mono project focuses on compilers and runtime libraries and does not directly provide an IDE. Developers must choose from a series of available tools the one best suited for them.
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Monodevelop is a Mono/C# Integrated Developement Environment for Linux and MacOS X.
In late 2003, a few developers from the Mono community began migrating SharpDevelop, a successful .NET open source IDE from System.Windows.Forms on Windows to Gtk# on Linux. Since Mono 1.0 Beta 2, MonoDevelop is bundled with Mono releases and is available from the Mono download page
MonoDevelop parses your C# and VB programs as you type them and is able to provide contextual completion of methods.
The Eclipse IDE can be used to edit C# code, but lacks completion of methods.
As you might expect, there are many development tools available to C# developers, ranging from bare-bones text editors to complex integrated development environments.
Using Emacs or Vim
Emacs and Vim are general purpose text editors, not IDEs designed for use with .NET and C#. However, you can use them to write software in nearly any language.
Vim supports syntax highlighting for C#, but not much else.
Recently a commercial IDE for Java and C# has been launched, you can find it here