How do you register a domain without a TLD?

So I found this website that is visitable and does not have a TLD? Anyone got any idea how to do this?

It does have a TLD - in this case the TLD is ac.

This is actually a special case. Usually a TLD does not have an A record associated with it:

$ host -t A ac.
ac has address 193.223.78.210
$ host -t A com.
com has no A record

To get this behaviour, you would have to register your own TLD.

New gTLDs can be registered like the recent .xxx for a small fee (US$185,000):

http://www.icann.org/en/topics/new-gtlds/strategy-faq.htm

It's likely you're using a dns 'default search domain' configured most likely via DHCP or a local hosts file setting.