Putting old hardware to new use is a good thing. Especially if the outcome is over 9000 times cooler then it's original purpose. People have came up with really nice and innovative ideas. Notably Groovers Laser Engraver which is the forefather of all the DVD Laser Engraver, CNC machines and recently designed 3D printers.
This is a collection of the most noteworthy builds people came up with. Here you will find instructions on
a draw bot
a Persistant of Vision System
a (Dot matrix) Printer
several laser engravers
a tiny CNC
and of course 3D printers.
But before you start you should take a look at the very first Instructable illustrating the disassembly process of CD and DVD drives. When it comes to LAZORZ be warned though. Those diodes are very dangerous if handled without proper safety glasses.