A - Agilent Technologies Inc.
B - Barnes Group Inc.
C - Citigroup Inc.
D - Dominion Resources, Inc.
E - Eni SpA
F - Ford Motor Co.
G - Genpact Limited
H - Hyatt Hotels Corporation
I - Intelsat S.A.
J - none
K - Kellogg Company
L - Loews Corporation
M - Macy's, Inc.
N - NetSuite Inc.
O - Realty Income Corporation
P - Pandora Media, Inc.
Q - Quintiles Transnational Holdings Inc.
R - Ryder System, Inc.
S - Sprint Corporation
T - AT&T, Inc.
U - none
V - Visa Inc.
W - Wayfair Inc.
X - United States Steel Corp.
Y - Alleghany Corporation
Z - Zillow Group, Inc.

Interesting post from 2005 on the turnover of single letter ticker symbols.