README

Stick With takes absolutely positioned elements and changes them to 
  fixed position elements before they are about to scroll off the top
  of the page. When the user scrolls back up they become absolutely
  positioned again.
  
  This has the effect of the element scrolling with the page until it
  "bumps" up against the top of the viewport. It stays there until
  the page comes back down and grabs the element again.

Options: top - (optional) the minimum number of pixels the element should
               stay away from the top of the viewport. default: 0.

Usage:
 $('selector').stickWith( {top: 20} ); 
 //Selected elements must be absolutely positioned before stickWith is called

See a demo: http://www.eecs.tufts.edu/~asamps01/stickwith/demo.html

TODO: Resizing the window is not handled at all. The element will jump
  sideways as you scroll past the place where it should switch to absolute
  positioning.