There are not many one-letter domain names that are owned by companies, and most of the companies are publicly traded stocks.
These companies were lucky enough to register them before December 1, 1993 (or buy them from someone who registered them by that date), as the Internet Assigned Numbers Authority put a restriction on single character domains at that time.
o.co and o.info are both owned by Overstock.com (OSTK).
q.com is owned by CenturyLink (CTL). If you go to q.com, it takes you right to CenturyLink's web site.
x.com is owned by x.commerce, which is owned by eBay (EBAY). When you go to the x.com website, it takes you to the x.commerce website.
z.com is owned by Nissan North America Inc., which is owned by Nissan Motor (NSANY), which trades on NASDAQ. However, the website is currently inactive.
i.net is owned by Future Media Architects, a privately held company. The company also owns c.tv, h.tv, k.tv and n.tv along with several other dot tv domain names.
u.tv is owned by UTV Media (UTV.L), a broadcasting and New Media company based in Belfast in Northern Ireland. The company trades on the London Stock Exchange.