Isotope is fairly nice, but it is still missing fluid width on the layout elements. In other words, although it does respond fairly well to horizontal resizing of the browser, it still doesn't do a good job of dynamically adjusting the width of elements horizontally while at the same time stacking them vertically.

I made some custom modifications on the original jQuery masonry plugin to get the effect I needed for my upcoming web app. You can preview it at http://test.bookflavor.com (This is an in-development sneak preview so don't expect everything to work perfectly, but you will be able to preview my personal modification of the jQuery masonry plugin.)