CoverFlow is another Apple acquisition, and was originally developed as a conceptual iTunes cover art browser:

The original application was able to use locally stored artwork (such as the Synergy database), but couldn’t retrieve artwork from DAAP servers.
Since its inclusion on iTunes (and its announced - and oft criticized - inclusion in the Leopard Finder), it has spawned countless variations.
Resources
JavaScript implementations
- MooFlow, with Ajax and JSON support.
- A non-animated one (complete with images and CSS for the scroll bar).
- An animated one that squeezes images horizontally.
- Another that dynamically zooms images rather than slanting them.
- SlideFlow, which requires you to pre-process images.