When it comes to grey the most obvious colour to pair it with is black. However, most greys that are lighter than charcoal work brilliantly with brown and khaki. A light grey suit paired with rich brown suede boots can make a powerful statement when done properly.
Another great combination you might want to try is pairing a grey jacket with khaki trousers, as exemplified by Italian designer Brunello Cucinelli. Cucinelli consistently manages to create effortlessly-looking smart yet casual outfits (note the cargo pockets on the trousers). I only wish I could afford to buy his stuff!
(Photos: Brunello Cucinelli)