There are a number of ASCII art editors available on the web, but most suffer from poor usability or small feature sets (one notable exception being eigenbom's awesome fork of ASCII Paint). For development of my own projects, I needed an application equipped with a wide range of tools for quickly drawing and manipulating ASCII art, as well as the ability to easily browse the images created as stored in their native format. Thus REXPaint was born.
Over the years since its first public release, REXPaint has found use as a general purpose ASCII art editor, as well as a roguelike development tool for mockups, mapping, and design. I love seeing what people create with this program, so send me a link/copy if you've made something cool! (Or share it with us on the forums)