I came across this video clip of the Russian Vityaz amphibious tractor, and was amazed by how well it crosses all sorts of terrain. See for yourself.
I'm used to offroading in Africa, where dust, rocks and sand are much more common than mud, snow and ice; so this sort of capability is new to me. If any readers have seen this thing in operation, or Western equivalents, I'd love to learn more about them. Please let us know in Comments.