Of the many sprues you get in the Ork Stompa box is this one with the gubbinz and krew.
It allows you to customise your Stompa with lots of bitz. There are lots of (probably) spare bitz that you could use on other Ork vehicles. I would like it if both Games Workshop and Forge World made more of these types of things for conversions and scenery. I know I would appreciate them, and I am sure others would as well.
See the full workbench feature on my Stompa.