Nothing counters Nasus. The cycle of life continues. He will farm, they will die.
In all seriousness the best counters to nasus are champs with some kind of ranged poke/stun and the ability to do some good burst damage. A lot of countering Nasus though lies in game skill not champion kit. Things like getting between him and low-health minions, answering every Q with a poke, etc. He honestly is pretty weak in lane until his Q gets farmed as E does virtually no damage. Just watch out for his W and his jungler (unless you are GP, then just eat an orange and it's kay).
TL;DR Force him to his tower, if he tries to farm his Q make him pay for it with some HP, watch out for his jungler trying to gank. Good picks: GP, Panth, Sion, Akali, Teemo