Qute is a very easy to use text editor with TeX and Markdown support. Qute offers per paragraph preview, i.e., users can switch between editing the source and viewing a rich text rendering with typeset formulas for each paragraph separately.
Qute is currently evolving to include support for user-defined markup languages and transformers via OMeta, see this post.
To make looking at a single text file for hours appealing, Qute offers switchable themes with subtle background images and font effects. Also, Qute includes a couple of great open-source fonts that are a joy to work with.
Qute's user interface is distraction-free and offers a full-screen mode. In this sense, it is similar to such great editors as WriteRoom, Dark Room and WriteMonkey. As far as the themes are concerned, however, Qute is far more adventurous.
Qute reads and writes plain text files using the Markdown markup language for rich text formatting and TeX syntax for formulas. Paragraphs are separated by blank lines.
While it is possible to edit arbitrary text files with Qute, editing, e.g., source code is not what Qute is intended for. Instead, Qute is a tool for writing prose.
Qute is built using web technologies. In particular, it uses Chromeless, Showdown and MathJax.