I personally wouldn't consider tubes to be plug-ins, because as MW has explained in his own way, a plug-in would be something that either

A) adds core functionality to an application that is not already there, or

B) Automates or simplifies an existing process in an application that would normally take more effort than the average user is willing to take to do.

I would be more inclined personally to classify them as PSP content, just as Poser files are Poser content.  I've been thinking that there should be a specific category on ShareCG where tubes and brushes for PSP and PS can go to make them easier to find and make them more distinct from the other 2D stuff.

Another classification might be "Add-on" but that's almost synonymous with plug-in.