It’s rather amazing how many of the single-letter domains have no websites behind them. The only ones with domains are q.com which goes to qwest.com, and x.com Paypal’s Developer Network.
a.com nothing
b.com nothing
c.com nothing
d.com nothing
e.com nothing
f.com nothing
g.com nothing
h.com nothing
i.com nothing
j.com nothing
k.com nothing
l.com nothing
m.com nothing
n.com nothing
o.com nothing
p.com nothing
q.com qwest.com
r.com nothing
s.com nothing
t.com nothing
u.com nothing
v.com nothing
w.com nothing
x.com Paypal Developer Network
z.com nothing
0.com nothing
1.com nothing
2.com nothing
3.com nothing
4.com nothing
5.com nothing
6.com nothing
7.com nothing
8.com nothing
9.com nothing

That’s only .com. There are of course other TLDs with one-letter domains like j.mp, t.co, and o.co. A little history on the one-letter domains: http://en.wikipedia.org/wiki/Single-letter_second-level_domain#Active_single-letter_domains
Ben, thanks for that link. Interesting history and reason behind the one letter domains. I also like that U Magazine tried getting U.com by designing a logo and rebranding their website.
I didn’t know that single and two-letter domain names were restricted, but it makes sense. I have always admired the simplicity of American Airlines aa.com url perhaps that admiration has led to my brand loyalty (That, and the free bag-check with gold member status!)
i think the Internet Assigned Numbers Authority (IANA) should have all the empty single letter/digit domains automatically forward to sesamestreet.com. C’mon, put those domains to good use for the public.