The Rasterbator is an application which creates rasterized versions of images. The rasterized images can be printed and assembled into enormous (or smaller, if you prefer) posters. Enter the online Rasterbation Gallery to see what the images look like.
The Rasterbator originated as a web application at homokaasu.org, but it has gained so much popularity that the web server occassionally cannot handle the load and a standalone version was in place.

The standalone version is the same as the web version, except that downloading images from the web and image cropping are not supported, and you have to set the output size numerically (number of pages wide/high) rather than using a fancy drag handle. The results are exactly the same.