Visual artist Alex Dragulescu has created a set of aesthetic, organic-looking visualizations of malware.

"For each piece of disassembled code, API calls, memory addresses and subroutines are tracked and analyzed. Their frequency, density and grouping are mapped to the inputs of an algorithm that grows a virtual 3D entity. Therefore the patterns and rhythms found in the data drive the configuration of the artificial organism."

Have a look at: http://sq.ro/malwarez.php

Update:

MessageLabs presents Dragulescu's visualizations in a more taxonomic fashion on this page.

Also, Dragulescu himself has made visualizations of spam in his projects Spam Architecture and Spam Plants. Good stuff!