Word processing was one of the personal computer's first killer apps, and it is still the cornerstone of any productivity suite.
Google Docs is the lightweight in the group, with the fewest word-processing features (even its find-and-replace function is marked "beta," which tells you something), but this is not necessarily a bad thing. If you like a clean interface, then Google's word processor is for you. Documents open by default in a new "fixed-width" view that's the equivalent of looking at the "page preview" mode of Microsoft Word.
And Docs does have some interesting tricks -- it lets you treat the document as a Web page and edit its HTML and CSS information, for example. Given that you can "publish" any Google document as a Web page with its own URL, this means you can create Web pages that you can update from anywhere without needing to FTP files to your Web site.
It takes a lot for a Web app to handle user interface issues such as formatting the on-screen display of type fonts and supporting the extensive file management that's a big part of word processing. All three of these word processors are capable of tasks such as formatting the typefaces, placing and sizing graphics, arranging paragraphs, and setting up tables. But only ThinkFree offers the really sophisticated features, such as letting you format a hanging indent. (This apparently has as much to do with the user interface as it does coding -- Google Docs and Zoho preserve hanging indents in imported documents, but there's no place in their interfaces for you to create one.)
In fact, ThinkFree's word processor gives you the most features (or the most unimportant ones, depending on how you feel about, say, drop caps), and the most control over things like the content of headers and footers.
Zoho Writer falls nicely in the middle. It offers more formatting control than Google Docs (it's easier to set up a header and footer in Zoho Writer, for example, than it is in either of the other suites) but it isn't as feature-heavy as ThinkFree.
Zoho Writer's support for Google's Gears is another plus. Writer added the feature late last year, becoming one of the first non-Google apps to work with the technology. Other recent Writer improvements include pagination, document headers and footers, an equation editor with LaTeX support, and more.
Google Docs and Zoho Writer both include "presence" features that show you who else is editing the document in real time. Zoho goes even further to support chat, allowing you to either broadcast messages to all other users, or click on an individual in the Collaborators list and open a private chat window.
In testing, ThinkFree was the only app that had noticeable performance issues, mostly because it seemed to need to download Java code almost every time a new document type was opened, or after its browser window was closed or the computer was rebooted. ThinkFree also seemed to be more at the mercy of overall network performance than either Google Docs or Zoho.
Picking a Winner:
Overall Google Docs wins by a nose over Zoho Writer. Both let you work offline with Gears, and both maintain version histories. Google Docs takes the lead with its leaner, cleaner user interface and unified file management. ThinkFree is a good choice for Microsoft Word fans, but its performance issues keep it in third place.