Google DocsGoogle Docs is simple to use and share. You can share the documents with certain Google users or the public. It's good for showing your works but may not be the best to keep track of who-edited-what since the main purpose of Google Docs is to provide an online alternative to MS Office. As a result revision options are not that structured.
WriteBoardWriteBoard is a product of 37Signals. As with most of their products, WriteBoard is super simple and easy to use. You can protect every writeboard you create with a password and username. Every WriteBoard has a URL that is almost unguessable. You can invite collaborators to share and edit Writeboards. Changes are highlighted in different colors, making it easy to keep track of the changes and their authors.
Mozilla BespinBespin is a beta application from the Mozilla Labs. Its simplistic approach to text-editing and sharing may find a lot of takers. But it may not be the best option for an average computer user. It uses various commands to do certain jobs which otherwise would have been easily done with simple UI. It's more like Nano for the web. Bespin allows you to do complex jobs like SVN controls or search-and-replace with regular expression. Perfect for geeks and Linux users.
Etherpad is open-source and can be run on your server. When Google announced Wave, many of its collaborative features were already available with Etherpad. Finally Etherpad was acquired by Google!
Etherpad is "the most frictionless way to get people on the same page" (Drew Houston, Dropbox). From colour hightlighting to online chat with collaborators, Etherpad has all the features you can ask from an online text-editor. You can even export your texts as HTML files. Etherpad has a time slider to keep track of times when certain changes were made. Creating a new pad is just a click away!
CollabEditCollabEdit is aimed for programmers working online. It supports syntax highlighting for a number of programming languages including C, Perl, PHP, HTML, XML, Visual Basic. Like Etherpad, CollabEdit supports chat among collaborators. You can download the document as a txt file. Its simplistic approach may find many takers among the programming fraternity.
Amy EditorAmyEditor probably has more features than any other online collaborative editors. The interface resembles Mac OS. AmyEditor supports syntax highlighting (of almost all languages), tons of keyboard shortcuts, templates of multiple languages, multi-tab editing, and tons of features. You can edit and add code snippets for frequent uses. It offers a console with logs. Perfect choice of programmers.
Tell us your choices in the comments section.