As an enthusiastic outliner I have compiled the following list of cross-platform, browser-based, free web outliners. Online outliners present a number of advantages over desktop applications (such as Ecco on Microsoft Windows, or OmniOutliner on Mac OSX), and are increasingly offering comparable functionality. These include platform, device and location independent access, collaborative functionality, as well as integration with other third party applications.
Hopefully this list will assist you in the eternal search for the best free outlining solution!
LooseStitch is a light weight outliner that has a free plan (pro accounts start at $24.95/year). The free version has all the basic functionality you would expect from an outliner, including the ability to import OPML, manipulate nodes, and print outlines in plan text. You can share your outline with other users to allow them to edit your outline.
The slightly odd interface enables editing using a pannel to the right of the main outline. This also offers the ability to colour code the text of individual outlines, although this is limited to four different colours.
The Lucid Outlining Application is a free outliner with a light weight interface that will appeal to those seeking to create simple nested lists. This outliner currently has limited functionality, but does offer a range of keyboard shortcuts to speed up the management of your outlines.
Lucid stores your outlines on your local machine, rather than in the 'cloud', but this does mean that you cannot access your outlines from other computers/browsers.
Targeted at the creation of reports and large academic documents, The Outliner of Giants is a free outliner that is integrated with Google Accounts. This outliner has all the basic basic functionality associated with outliners, as well as the ability to export your outline directly into to Google Docs.
The interface is focused on editing larger outlines, so is not good for the fast creation of the simple task lists typical of other outlines.
With refined collaborative capabilities, thinklinkr allows multiple users to synchronously edit an outline. It is a feature rich outliner, with the full range of functionalities including the ability to import and export outlines, track revisions, and switch between different outlines. The interface is well developed and is supported by an extensive range of keyboard shortcuts.
However, some may find the interface a little cluttered and the range of functionalities a little confusing for many outlining tasks.
All functionality referes to the free version. Some have additional functionality with a subscription.
If you know of any other free, fully functional, web-based outliners please let me know.