Since Microsoft released some of the .NET framework code as open source, some great things are starting to happen.
Recently Omnisharp was released. This is a great plugin for Sublime Text, Vim, Emacs and more. It adds most of the needed functionality to convert it to a viable IDE for Unity.
Bundle this with the broad range of plugins available at Sublime Text’s Package Control you can get a lighting fast, top notch IDE on any platform!