jQuery plugin to create sticky headers
At the risk of nitpicking there's quite a fiew small/micro-optimizations that could be done:


group multiple var declarations using commas

var containerTop = container.offset().top, headerOrigin = header.offset().top;
chain method calls to the same object

header.before(placeholder).css('...')
group multiple calls to css() passing an object as the parameter:

header.css({ position: 'fixed', width: originalWidth + 'px'});