Dock Menus lets you create multiple free-floating docks separate from the built-in Mac OS X Dock; the floating docks can be moved around your desktop as needed. By creating multiple docks, you can group related apps, files, and folders. Dock Menus works with Mac OS X Leopard or later and costs $5.
From the same developer as Dock Menus, iDock works with the built-in Mac OS X Dock, letting you create multiple Docks, each with its own content, and switch among them as needed. This creates an effect somewhat similar to the multiple home screens in iOS and the Launchpad preview; iDock works with Mac OS X Leopard or later and costs $5.
Like iDock, Dock Spaces lets you create and switch among varying Docks using the traditional Mac OS X Dock interface. Dock Spaces goes a step further than iDock in that it offers integration with the Mac OS X Spaces feature, allowing you to tailor virtual desktops that automatically open specific applications, specific windows, and a specific Dock for a variety of tasks such as graphic design, document editing, social media, Web browsing, or chat. Dock Spaces is free and works with Mac OS X Leopard or later.
DragThing is a venerable Mac tool that predates Mac OS X. It allows you to create Dock-like work areas that can contain shortcuts to applications, folders, files, and URLs. There's also a space to store copied items for later pasting, offering a way to copy many items and have them readily available. DragThing costs $29. The current version of DragThing supports Mac OS X Tiger or later, but earlier versions are also available for earlier versions of Mac OS X.
Quicksilver is a free Finder alternative for Mac OS X Tiger and later; it lets you launch applications and locate specific files quickly just by typing the first few letters of an application or file name. It's a simple, keyboard-centric way to launch apps and open files. Like Alfred, it learns your preferences and orders results accordingly, and it lets you assign keyboard shortcuts to a wide variety of actions.
Mission Control looks like it will be an interesting combination of existing Mac OS X features -- Spaces (Apple's virtual desktop feature), Exposé (which allows you to see thumbnails of all Spaces, open windows, and items hidden by windows, and to switch apps), and Dashboard (a feature that allows easy viewing of a range of widgets, or tiny applets) -- in a single interface.
In bringing these elements together, Apple is trying to offer a one-click view of all running apps, windows, full-screen app views, and spaces. The ability to swipe through all these items will borrow from the iOS ability to swipe across multiple home screens.
In the new iteration, Dashboard appears to have its own Space or full-screen view instead of appearing as an overlay to the desktop as it does currently. While I'm not enamored of that particular change, overall I think the Mission Control concept is solid as a way to quickly see everything that's running on a Mac and to easily switch to the tasks you need.
I'm not aware of any existing tools that match the complete integration of these features that Apple is promising in Lion, but here are three that offer useful enhancements to Spaces and Exposé along the lines Apple seems to be planning -- and which may even be better than Apple's ultimate Mission Control solution for some users.