Almost any bike can be used for commuting, as demonstrated by the wide variety here. My suggestion? Shop for a local shop first. Find the best bike shop in your area and buy from them. By best, I do not mean most expensive, or most chic. I mean the shop where they pay attention to you, try to figure out what you need and want, not what earns them the biggest commission. Then, see what they have in your price range. Unfortunately, $600 isn't a lot to spend on a new day-in day-out commuter. But most of the good manufacturers make something quite serviceable in that price range -- Kona Dew, for example.