If you are a developer who prefers using Linux as your platform but is required to churn out applications on .Net or some other non-Linux platforms OR if you are a Mac user who needs to release .Net applications, then MonoDevelop is for you. In a nutshell, MonoDevelop is "is a GNOME IDE primarily designed for C# and other .NET languages. "
Whilst it is geared towards .Net application development, MonoDevelop also has support for several languages, with C#, VB.NET and C/C++ support included, and Boo and Java (IKVM) support available as separate add-ins. I do not normally encourage development for .Net on the Linux (or even Mac) but sometimes you need to do it to earn a living. :)