jQuery Stickicky gives you very smooth sticky divs with just a single line of code. Available but broken solutions like StickyScroll have problems in many browsers with flickering. Stickyicky is smooth in Chrome, Firefox, Safari. We haven't checked IE yet.
I originally learned about this technique from Derek Allard on his blog here.
To use it, load the jQuery file, and call it like so:
$('#sticky_box').stickyicky();
If you want to add an offset to the top, you can do that by passing the number of pixels to offset by to the function:
$('#sticky_box').stickyicky(50);
The above will trigger the stickyness at 50 pixels below the top of the browser window.