Molokai is a Vim port of the monokai theme for TextMate originally created by Wimer Hazenberg.
By default, it has a dark gray background based on the version created by Hamish Stuart Macpherson for the E editor. Screenshots are available here.
256-Color terminals are also supported, though there are some differences with the Gui version. Only the dark gray background style is supported on terminal vim at this time.
Just copy the file on your .vim/colors folder.
If you prefer the scheme to match the original monokai background color, put this in your .vimrc file: let g:molokai_original = 1