ThinkFree Office Online
ThinkFree has three main components — Write (a word processor), Calc (a spreadsheet), and Show (presentation software) — all of which are listed on the site’s My Office page, along with your most recently edited files. If you’re concerned about document compatibility with Microsoft Office, you want ThinkFree. There’s simply no contest.

 
Google Docs & Spreadsheets
Google Docs is a free, Web-based word processor, spreadsheet, and presentation application offered by Google. It allows users to create and edit documents online while collaborating in real-time with other users. Google Docs combines the features of two services, Writely and Spreadsheets
Documents, spreadsheets, and presentations can be created within the application itself, imported through the web interface, or sent via email. They can also be saved to the user’s computer in a variety of formats. By default, they are saved to Google’s servers. Open documents are automatically saved to prevent data loss. Documents can be tagged and archived for organizational purposes.

Collaboration between users is also a feature of Google Docs. Documents can be shared, opened, and edited by multiple users at the same time. In the case of spreadsheets, users can be notified of changes to any specified regions via e-mail. The application supports popular Office file types such as .doc or .xls. It also includes support for the OpenDocument format

Zoho Office Suite
The Zoho Office Suite is a web office suite which includes tools for word processing, spreadsheets, presentations, databases, note-taking, wikis, CRM, project management, invoicing and other applications. Being web-native, all Zoho applications are operating system independent.

The applications can be used either to create content or to manipulate files created with other widely distributed office suites. Zoho can read and write file formats used in Microsoft Office, OpenOffice.org, and other systems. The programs in the suite can be used individually but offer additional integration benefits when used together. The data (document) files can be saved on Zoho’s servers as well as those of several integration partners like box.net or Omnidrive, which allows synchronization back to the local PC.

Writeboard
Writeboard is a free collaborative (but not real time) text editor, which allows creation of an unlimited number of web-based text documents. Each Writeboard has a separate user name and password, and changes can be monitored via an RSS feed.

Writeboard is a very simple application but supports Diff, allowing users to compare changes made to the document. Writeboard has since been integrated with both Basecamp and Backpack.

ajaxWrite
ajaxWrite is a web-based word processor that can read and write Microsoft Word and other standard document formats. Anytime you need to open, read or write a word processor file, simply point your Firefox browser to www.ajaxwrite.com and in seconds a full-featured program will be available for you to open, edit, print and save.

ajaxWrite has been designed to look like Microsoft Word, making it easy for anyone to start using it without needing to learn a new program. ajaxWrite also handles all the popular document formats so it’s easy to share your files and collaborate with your co-workers and friends. Once finished with your document, you can easily save your work right to your hard drive. This keeps you organized and works in the same way that you’re already accustomed to.

ajaxWrite works from a Firefox web browser on any operating system and on any device, no matter where you are or what computer you’re using. And because the application itself lives on the web, we handle the updates automatically so that you don’t have to deal with costly upgrades or getting stuck with old versions.