Update: These instructions are out of date. You should instead install MonoDevelop from source.
Monodevelop is probably the best IDE for developing .NET applications on Linux. In order to install and use it for development, you will also need the Mono .NET runtime environment installed. Thankfully, installing monodevelop and mono are very simple on Ubuntu.
Run this command from a Terminal window:
sudo apt-get install mono mono-gmcs mono-gac mono-utils monodevelop monodoc-browser monodevelop-nunit monodevelop-versioncontrol
You will now have a shiny new Monodevelop installation, along with the documentation, nunit plugin, and version control plugin. You are using version control, aren’t you?