Lightbox or Colorbox. I would say colorbox, its successor to thickbox, which i always liked, its smarter and just feels better. Maybe Lightbox has more features, but i see its missing one i really need and like, which is "Per post gallery" so all images on the post/page can be looked at with modal view(that black screen that goes over the content) continuous. Also it has option for login screen and has better looking default theme.