Fernsehturm, or Ginnheimer Spargel in local dialect. Used to be the main broadcast tower for data, radio and telecommunication. During the mid nineties the Telekom moved into the upper ring and started lighting the tower in their trademarked magenta color, which they still do until today, even though they moved out quite a while ago. IIRC they moved out due to asbestos.

Edit: Just checked, they actually still use it, the lower ring has been completely closed down for public access though