Interesting short urls

Have you ever wondered what are the shortest URLs?

Found the following 1 letter second level domain names with .co and .com TLDs. Interesting list.


a.co => Redirects to amazon.com
o.co => Redirects to overstock.com
g.co => displays page mentioning about it being used for Google Product Links
s.co => __ NO_REDIRECT__ Startup America Site
t.co => displays page mentioning about it being used for a twitter service
x.co => Redirects to app.x.co  GoDaddy site for URL shortening

X.com => __ NO_REDIRECT__ An ebay Product
q.com => Redirects to Centurylink.com

Now taking a.co example, a.co redirects to amazon.com. but amazon.co redirects to an help page in amazon.com. Also surprising that they have a short URL for a.co but not for a.com

Also interesting why companies like apple, microsoft, intuit doesn't have single letter domain urls. i.co or i.com and m.com likes lucrative URLs for them to own for more web hits... doesn't it? (Though have to agree that the number of people actually typing URLs is ever decreasing with more and more usage of Google Chrome Awesome Bar :) )

Thoughts?

Also, if you are confused on what are TLD, Second level domain names etc. and their relationship to URLs, here is a quick reference:
http://www.seomoz.org/img/upload/anatomy-of-a-url.jpg