More than just a static model. This Metal Slug tank from the SNK series of the same name rocks!
The amazing Omniwheels allow it to spin circles while driving straight. When used as regular wheels they have friction. However, when sliding sideways they do not. Check out the 1:08 video on YouTube - linked below.
Yes - this is just one "wheel" as we normally call it. Original omniwheel from Sariel (yes - same one in my favourite designers links). mahjqa has replaced the gear wheels with normal LEGO tyres and wheels
More photos at mahjqa's Flickr page. Click on this link: http://www.flickr.com/photos/28134808@N02/sets/72157625017384671/with/5071779759/
Also amazing YouTube video of the Metal Grudge in action at this link: http://www.youtube.com/watch?v=fOuP_w0FZi0