Single-letter second-level domains are domain names in which the second-level domain consists of only one letter, such as x.org.

Such domains are rare, as on December 1, 1993, IANA explicitly reserved all single letter and single digit second-level domain names in the TLDs .com, .net, and .org.

However, the few such domains that were already assigned were not recalled, and so circumvented the restrictions that have existed since.

i.net INet Corporation
q.com West Communications
q.net Q Networks
x.com PayPal
x.org The Open Group
z.com Nissan

[Source Wikipedia]