I grabbed a couple of images from around the web of something neat on the shelves at the recent Forge World Open Day. It's a bunch of little gretchin-crewed tanks. I haven't looked at my Ork codex in a while, so I don't know what these would stand in for, but I'm sure I could shoehorn them in somehow. I love Snotlings and Grots. They're like the unruly trailer park kids of the Orc/Ork world. From what I hear many of the parts on these little tanks are interchangeable, so you can customize them to a degree. Images were taken from this thread on Warseer and the GW daily blog.
I've also seen some photos of variant parts for the Ork Stompa kit and some other bits and bobs for other armies. But these little tanks are at the top of my Shiny List at the moment.