This is good fun, a class written in PHP which can take the URL of any image on the Internet and generate an ASCII art version of the image on the fly, check out this image of Cerro Torre from Patagonia as an example, then try another image for yourself:

Cerro Torre in ASCII : http://ascii.techhappens.com/index.php?url=http%3A%2F%2Ffarm1.static.flickr.com%2F14%2F18481774_f4c0580f17.jpg%3Fv%3D0&quality=2&size=2&color=on&submit=Submit

Impressive stuff!

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Updated 2021 : note that the above post is out of date as it was originally published in 2007, but is left here for archival purposes. Unfortunately the external link is now dead, so I have unlinked that.