The best replacement for shadowbox is lightbox.

The best alternatives to shadowbox are rated as following:
★★★★★  lightbox
★★★  midnight
★★  thickbox
★  fancybox

Your alternative for shadowbox is not listed? Suggest!
Speaking about shadowbox, one probably shouldn't forget about vimeo.

Other search terms related to shadowbox and their ratings are:
★★★★★ vimeo
You have in mind a companion for shadowbox and it is not yet here? Propose!



Google Trends

Scale is based on the average worldwide traffic of shadowbox in all years. Click on graph for more.
shadowbox
lightbox
midnight