Create.js is an open source, JavaScript-powered web editing interface for content management systems.

The interface focuses on simplicity and aims to enable the user edit the content without leaving the page but complete the edits directly on the viewed content.

It has support for the plain contentEditable, Aloha, and Hallo editors (more to be added) and, various configuration options exist to customize the behavior of Create.js.

Redactor is a JavaScript-powered (jQuery) WYSIWYG editor with a clean and good-looking interface.

It auto-converts textarea fields into an editor and the editor is capable of all the major actions (text-styling, adding image/file/video/tables/lists).

The uploading of files and images makes use of HTML5 APIs and they can be uploaded via drag 'n' drops besides selecting files.

Redactor can work inline or fullscreen, there are many settings to customize it and it is multilingual (already supports many languages).

P.S. The website of the resource is half English and half Russian but it is coded in English and demos + docs help understanding how it works.

WYSIHTML5 is an open source rich text editor, developed by Xing, that produces non-bloated, valid HTML5 code.

The editor doesn't rely on any JS framework,  it is lightweight and loads/works pretty fast.

It transforms textarea fields and provides the major stuff including text styles (bold, italic), lists, headers, inserting images or links and coloring the text.

There is speech-input (which is only displayed in Chrome) and HTML source code can be viewed with a click.

WYSIHTML5 speeds up the writing process by auto-linking the URLs and offers a secure usage by using sandboxed iframes in order to prevent identity theft through XSS.

Mercury is a HTML5-powered WYSIWYG editor similar to the popular ones like TinyMCE or CKEditor but with a different usage experience.

Rather than being positioned inside a given element, it appears over the complete web page and can be used to edit the whole page or any number specified areas.

It supports previewing the edited content and inserting links, images, videos or tables. File uploads can be accomplished with drag 'n' drops.

Snippets can be defined and inserted quickly with the help of a sidebar. And, a similar sidebar exists for taking notes.

Also, collaborative editing is supported. Just edit any page that others are working on at the same time and see their changes in real time.

Mercury can be either installed as a Rails gem or by including the necessary JS and CSS files into our web pages.

Ace is an open source, standalone and web-based code editor that focuses on providing a similar or better experience with desktop-based ones.

It is actually developed for the well-known Cloud9 IDE and can be integrated into any web page easily.

The editor's syntax highlighting works with JavaScript, HTML, CSS, JSON, PHP, Perl, Python, Ruby, etc..

There is support for auto indent/outdent, completely customizable key bindings ad theming.

Users can perform search/replace operations with regular expressions besides the standard method, toggle between soft tabs + real tabs and much more.

Maqetta is a web-based WYSIWYG application which focuses on easing the process of creating HTML5 web pages.

The project is open source, does not require any plugins to run and can be used as a hosted service or by installing locally.

Its interface works with drag 'n' drops, there is support for desktop-like functions like undo/redo, preview in browser, save/save as and much more.

This version of Maqetta has the best support for Dojo library and enables you to insert the widgets/controls (like buttons, accordion menus, calendar, etc.) of the library instantly.

The application makes getting feedback for the pages developed easier with the ability to create multiple reviews (for ex: for each version). The developer simply selects the pages to be reviewed and the system automatically sends e-mail invitations to reviewers. And, reviewers can either share their thoughts as text comments and/or on-screen graphical annotations (rectangles, ovals, arrows and text).

Maqetta has much more integrated features like the ability to create wireframes with the help of a sketch theme, mobile authoring tools and more.

CodeMirror is a JavaScript library that can convert any element on a web page into an interface for editing code.

It supports most of the popular languages (HTML, CSS, PHP, Python, Ruby, Java, C#, SQL, XML, etc.) and works cross-browser.

There are many options offered like enabling undo, line numbers, the behaviour of "tab" and "enter" commands.

Also, with the help of its API, the editor can be totally manipulated.

elRTE is an open source and feature-rich WYSIWYG editor that is built using jQuery UI.

It performs all standard functions of an editor and has all the extras like undo-redo, advanced table management, creating a custom HTML element with a custom style + inserting it quickly and more.

The interface is controlled via a single CSS file and can be customized pretty easily.

It has a built-in "save" button which can be attached to a JS function that saves the content that is pretty useful for helping users not to lose their content.

elRTE can be integrated with any file manager, however, there is elFinder, a file manager that has ready-to-use integration (can be used standalone too).

CLEditor is a lightweight jQuery plugin for adding WYSIWYG HTML editors on web pages.

The plugin works by converting a standard textarea element and it is cross browser.

It is fully customizable like adding/removing/updating controls, enable/disable the use of CSS and much more.

Also, the WYSIWYG editor can be controlled with a set of methods provided.

CLEditor is very well-documented and comes with optional add-ons like "table plugin (visually displays tables inserted)" and "icon plugin (for seeing the smileys inserted)".

TinyEditor is a lightweight (8kb) JavaScript WYSIWYG editor that is free to use in both personal & commercial projects.

It is a standalone script (does not require any JS frameworks) & converts any given textarea to an editor

The can be easily customized by updating the parameters (like removing some of the controls). And it is possible to style it via CSS:

TinyEditor can be resized, a starting content can be defined, set to generate HTML or XHTML code & more..