MonoDevelop from: http://monodevelop.com/
There is no equivalent to Visual Studio. However, for writing C# on Mac or Linux, you can’t get better than MonoDevelop.
The Mac build is pre beta. From the MonoDevelop site on Mac:
The Mac OS X port of MonoDevelop is under active development and has not seen a stable release yet. Recent work described by Michael Hutchinson has focussed on improving the usability and stability of Monodevelop on the Mac. This work will be released in MonoDevelop 2.2. Right now it’s not finished, and is very much an alpha.