After the Wachowskis saw his vision of dystopian Los Angeles in Frank Miller's comic miniseries Hard Boiled, they approached Geof Darrow to work as a conceptual designer on their Matrix films. Darrow's visual fingerprints are all over life outside the program, and he's responsible for some of the most iconic images in the film, including the battery farm where Neo wakes up and the insectoid robots. Even divorced from the films, Darrow's concept images are delightful to look at, filled with the incredibly detailed settings of unplugged life and the intersection between biology and the series' machines.
Darrow also provided the art for the Wachowskis' own comic for The Matrix Comics anthology, "Bits & Pieces of Information," which was later animated as part of The Animatrix. These images appear to be taken from the now out-of-print Art of the Matrix, but you can always follow Darrow on Tumblr, where he posts his art, including, on occasion, Matrix concept art. (Head over there now to see more from this series of recently posted Matrix art.)
Not all of his work is meant for the worlds of gritty and grim, however; Darrow also worked with Miller on the somewhat more family-friendly The Big Guy and Rusty the Boy Robot.
Some concept artwork for The Matrix [Geofrey Darrow via Brian Michael Bendis]