Quay extends the Mac OS X 10.6 (Snow Leopard), 10.7 (Lion), 10.8 (Mountain Lion) or 10.9 (Mavericks) Dock to show extended popup menus for most Dock items, including applications, stacks and URLs. Quay also allows you to make folder proxy icons with easily customized icons.
Mac OS X 10.5 (Leopard) introduced "Stacks" in the Dock, with a unique grid display. Starting with 10.5.2, Stacks once again can show hierarchical menus for their contents.
So why should you use Quay at all? Extra information, more flexibility. For one, the Dock's popups are limited to about 500 items; Quay's limit is in the tens of thousands. You can have a Quay popup on both sides of the Dock; Apple has them only on the document side. The Dock doesn't follow aliases/symbolic links in its menus; Quay does. It works for Smart Folders, also shows application versions and (for running applications) CPU and memory statistics. Still, if for some reason you want the Dock's normal display, just hold the Option key down before clicking.
Finally, I'm more likely than Apple to, on short notice, implement features you request.
Quay does no magic (but it does use complex technology superficially indistinguishable from that). It doesn't hack the Dock or the system. It uses no undocumented APIs, although it does depend closely on the Dock's implementation details.
What is new in this release:
Quay is now compatible with Mac OS X 10.9 (Mavericks).
Without registration, Quay pop-ups will appear only for the first item on each side of the Dock.