If I had my way there would be no obvious composers, e.g. most of Polyphonist's suggestions for the letter B ;-)
Here are a few less obvious composers who wrote for the
piano (or keyboard):
A - Thomas Ades
B - Amy Beach, Arnold Bax
C - John Casken, Ignació Cervantes, Abram Chasins, Domenico Cimarosa, César Cui
D - David Diamond, Henri Dutilleux, Vincent d'Indy
E - Enescu, Michele Esposito, Antonio Estévez
F - Louise Farrenc, Arthur Farwell, Samuel Feinberg, Field
G - Niels Gade, Hans Gál, Baldassare Galuppi, Giorgio Ghedini, (Glass, Gottschalk, Griffes....)
H - Roy Harris, Henselt, Honneger, Hovhaness
I - Ibert, Ilyinsky, Ireland
J - Rafael Joseffy
K - (Kabalevsky, Khachaturian, Kodály), Krenek, Kurtág, Miloslav Kabelác
L - Joseph Lamb, Philip Lasser, Ernesto Lecuona, Jón Leifs
M - Benedetto Marcello, Czeslaw Marek, Merikanto, Milhaud, Marjan Mozetich, John Musto
N - (Nazareth, Nyman, Nielsen), Per Nøgård
O - Nikolai Obukhov, Leo Ornstein, Henrique Oswald
P - John Knowles Paine, Palmgren, Domenico Paradies, Horatio Parker, Octávio Pinto
Q - Quilter
R - (Raff, Rautaavara, Rodrigo, Rota), Vladimir Rebikov, Roussel, Nikolai Roslavets
S - Kaija Saariaho, Harald Sæverud, Schnittke, Scharwenka, Erwin Schulhoff, Steven Stucky, (Sinding, Sorabji, Stanchinsky, Stenhammar...)
T - (Tailleferre, Taneyev, Tippett), Alexandre Tansman, Tcherepnin
U - Ustvolskaya
V - Fartein Valen, Vierne, Vorisek
W - Weinberg, Wiklund
X - Xenakis
Y - Benjamin Yusupov
Z - Judith Zaimont, Domenico Zipoli, Zemlinsky
. 
