Some of these folk also modify their computer cases. These are the case-modders. They often start out with colored cases --- acid green, fuschia, bright orange --- and then add cool grilles, special case lights behind the fans, etc. ...
A subset of case modders are sick creatures.
Like the freakazoid who made this: 
This is a no-case system, with a handle in the top. For more, check out these pictures.
Or, for art's sake, try this: 
This is called "Intel's Silicon Rising."
"Project Silicon Rising a commissioned piece for Intel Corporation has been completed. Out of a silicon crystal base we have a metalic winged figure taking flight at a dramatic angle. With the use of internal lighting in both the base and the figure, this has made for an interesting case mod. Shades of airbrushed metalic paint bathed in a crystal clear, protective layer of resin, all playing and reflecting light while accenting shadow highlight this piece. My very special thanks and love to Wendy Brackman, artist extraordinaire for her incredible sculpting and help on this project."
For more, check out these pictures.