There are only 3 One Letter .com Domains.
December 2 1993, at a time when the web was being formed, Dr Jon Postel registered all still available 23 single letter Domains and assigned them to Internet Assigned Numbers Authority (iana.org). It was Jon Postel’s intention to avoid a single company commercially controlling a letter of the Alphabet.
However, 3 domains were already registered:
q.com was owned by Qwest Communications, which was acquired by CenturyLink in 2010. As of this writing, q.com forwards to CenturyLink High-Speed Internet.
x.com was owned by Weinstein & DePaolis and was the homepage of a Netscape employee named Robert Walker. The domain was acquired in 1999 by the x.com financial services company owned by Elon Musk. X.com eventually became Paypal and Musk was forced out of the company. However, the x.com domain name was reportedly purchased back by Elon Musk in mid-2017, terms were not disclosed. The domain transferred out of brand protection MarkMonitor to GoDaddy under privacy in July, 2017. Elon Musk tweeted, “Thanks PayPal for allowing me to buy back X.com! No plans right now, but it has great sentimental value to me.” As of this writing, x.com retrieves just the letter x.
z.com was owned by Nissan -now it is the GMO Internet Group, Japan’s first ICANN-acredited domain name registrar in Asia (InterQ). The reported sale was for $6.8M US in 2014. The fist mention of z.com under the GMO Internet Group is from March, 2015: GMO CLICK Holdings local British company launches first service under GMO Internet Group global brand Z.com.
All other one letter .com domains are unresolvable. However, as published on Domain Name Wire back in 2011, trademark applications were filed to the USPTO in 2005, shortly after news got out that ICANN might distribute single letter .com domains, which were previously restricted. This hasn’t happened and many of the trademark applications are now dead.
The availability of one character domain names in TLDs varies, but only six exist in legacy generic Top Level Domains (Q.com, X.com, and Z.com, I.net, Q.net, X.org).
Some other gTLD single-letter domain names are also in use, usually as shortcuts:
a.co: Amazon.com – Amazon’s official URL shortcut. Generally used by Amazon in SMS messages for informing purchasers of activity on an order
a.org: AutismAwaremess.com – Awareness project coming soon by AutismAwareness.com
b.org: benevolent.net – Shortcut to benevolent.net
g.co: Google – Google’s official URL shortcut
m.me: Facebook – Facebook Messenger’s official URL shortcut
t.co: Twitter – Twitter’s official URL shortcut
t.me: Telegram – Telegram’s official URL shortcut
w.org: WordPress – Redirects to wordpress.org