Qute for PC/Mac is a text editor with Markdown and TeX 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. 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.
