I use oil-dry from any auto parts store, or kitty litter works good too, only for the initial, really wet spills. I use gasoline and some rags after that. Like Willy's says, if you've got a natural gas, propane, or fuel oil water heater, be sure to have the garage door open to prevent those not wanted explosions.
If you want it really clean, use muratic acid, but it will etch the concrete and your feet, hands, and lungs. You'd need knee boots, rubber gloves (thick) and a respirator for this stuff.
I just leave mine alone til it bothers me a bit. I figure it is just going to get dirty again and I don't need a yuppie type pretty garage floor anyway.