ASCII Web Artist is a web-based CGI application which generates ASCII HTML art from images. Just select an image and press submit. Please note that there is a 12KB/6500 pixel limit on uploaded images in order to preserve server bandwidth and horsepower.
If you need a sample image to try out, you can always use this one:
A Mac OS X application is also freely available for download.
If you want to check out the code for this application, you can download the ASCII Web Artist CGI mechanism. To host it, you must be running a Mac OS X server.