Ross, Alex Easter Egg - Watchman Watches Superman!

Check out the Kingdom Come trade paperback, available from DC and in any good comic-store. Look at the second panel on page 85 and you'll see a visitor from Alan Moore's seminal work, Watchmen, standing near a table on the left-hand side. It's Rorschach!
Alex Ross, who drew/painted Kingdom Come often inserts famous chracters from other works into his own to pay homage to other artists and writers.