Custom field for Custom Metaboxes and Fields for WordPress (https://github.com/WebDevStudios/Custom-Metaboxes-and-Fields-for-WordPress). This requires jQuery UI for drag & drop and sort functionality (http://jqueryui.com/download/#!version=1.10.4&components=1110111110000000000000000000000000). The post IDs are saved in an array, which can be rearranged by dragging and dropping posts in the attached posts column.