The recommended development IDE is MonoDevelop. This IDE is open source software under the MIT license, and has versions available for Linux and Windows. Currently it looks as if support for use on Macs is also in the pipeline.

On Windows systems you can also use Visual Studio, although the range of features available on the free Express versions may be limited compared to open source IDEs. You could use the commercial versions of Visual Studio (2005 and upwards), but this is generally not recommended since not all contributing developers will have access to these versions. For example, if you send me code which can only be compiled with the professional edition of VS2008 I won't be able to use it.