When making any sort of web application, a backend interface is a must-have, and nothing makes this more user-friendly than a non-HTML implementation for the text areas in the interface. Below is a list of 5 different, solid content editing methods - each with their own style and featureset; and best of all, they are all free. Without further ado, here is the list (in no particular order).
1. Nice Edit

NiceEdit is a standalone JavaScript WYSIWYG implementation that uses some of the beautiful FamFamFam Silk Icons for its interface. It takes simple textareas and converts them to a fully functional and familiar interface with a fair bit of functionality. The full panel includes text decorations, paragraph alignments, WYSIWYG lists, font sizes, font families, indenting, support for images, links, subscripts and superscripts, horizontal rules, a format remover, and a view for HTML source editing.
2. FCK Editor

The FCK Editor is very similar to NiceEdit minus the "lightweight". FCK is a solid (and large) editor with a ton of customization options and buttons. FCK, unlike NiceEdit, is not meant to simply create content for the web; rather, the FCK editor aims at bringing the feature-rich environment of desktop word-processing to the web through JavaScript. When fully configured, it has buttons for editing the source, saving, adding pages, previewing the page, selection of templates, copy, cut, paste (supports pasting from MS Word), undo, redo, find, replace, select all, remove format, forms, checkboxes, radio buttons, text fields, text areas, selection fields, buttons, image buttons, hidden fields, text styling, subscripts and superscripts, blockquotes, lists, indentations, links, pictures, Flash-based content, tables, smileys, printing, and spell-check.
3. TinyMCE

TinyMCE is probably the most popular and the most widely recognized JavaScript-based WYSIWYG editor around. It is used extensively in many Content Management Systems and has a very robust feature-set that rivals the FCK Editor. It has all of FCK's features, but has a plugin library and a wide userbase. A few extra buttons that TinyMCE has on top of FCK's offering is a button for non-breaking spaces, special characters, page breaks and a full-screen mode.
4. MarkDown

While Mark It Up adds HTML tags, MarkDown uses its own unique markup system that is easily understandable and requires much less work to both read and edit. It is a natural flow that is entirely text-based and does not use any sort of interface for publishing. This is a similar approach to Wiki Syntax, but (in my opinion) easier to learn and understand. It is hard to explain before you see it in action, so pay the Daring Fireball a visit.
5. Mark It Up

Where TinyMCE, FCK, and NiceEdit are WYSIWYG, Mark It Up is more of WYPISWYG (what you press is what you get). Mark It Up doesn't entirely ditch HTML, but makes using HTML as an editing format much easier. Instead of actually rendering changes made to the text inside of the text area, Mark It Up simply shows the tags, which is very useful if you like fine-tuning the HTML content of the content. It also works very well as a complement to blog comment forms due to its small set of buttons. It is based on jQuery, which is a put-off for some, but a major benefit for others (all dependent on whether or not you already use jQuery - chances are, you do). But wait! There's more! Mark It Up even works as a BBCode, Wiki Syntax, Textile, Dotclear, and Markdown Syntax editor.