Multiselect HTML select elements are hard for users. Selecting more than one requires Ctrl+clicking (or Cmd+clicking on Mac), and explaining this to users who sometimes have trouble even clicking the mouse at all is no one's idea of fun.
Better Select module overrides all multiselect HTML elements in Drupal and replaces them with checkboxes, in an auto-scrolling div so they don't take up much room on the page.
The first beta was released for Drupal 5.x and 6.x.
Drupal 6.x and 7.x are currently supported.