Several single-letter domains were purchased before the IANA reserved all of the remaining ones in 1993. See: http://en.wikipedia.org/wiki/Single-letter_second-level_doma...

A glitch in the matrix many many years ago. :-)

x.com was Elon Musk's precursor to Paypal.

X.com used to be a bank back in the day, my first bank I got while in college as it happens. They bought paypal (I think it was that way, not the other way around but I may be mistaken) and killed the bank. X.com just resurfaced a few years ago in its new form.

There are a few others out there, like http://q.com/

They've had x.com for many years.