Muriatic acid. It's nasty stuff, the same chemical used in pools to keep 'em clean, but undiluted it'll take everything but the concrete up. Should be available through just about any hardware or home improvement store.
Be careful when you use it! Like I said, it's really nasty stuff. Always keep something between you, your clothes, and it; like heavy rubber gloves, safety goggles, etc.
If your state doesn't allow the purchase of it without a hazardous material license, then go for a good quality engine degreaser and a stiff brush.
