Well, subdomains I know of. But it simply through me off because of the www. I guess if you think of it though, www could be considered a subdomain, or maybe the "maindomain", eh?
A co.uk is different, methinks. Every country (pretty much) get's its own top level. There's .uk .au .tk , ect... But doesn't the UK divide it into .co.uk .org.uk .gov.uk ect. ?