Select Multiple is a user-friendlier drop-in replacement for the standard <select> with multiple attribute activated. It is a jquery plugin. The source code is fully open source, avaliable under the MIT license.
It has a good design with check mark instead of that irritating traditional checkboxes. It works in an unobtrustive fashion.
You can use some callback functions, for example to show alert messages when the items are selected.
And there is one more thing you would like about select multiple. I has gem for rails 4 as well.
