There are some stump-rotting products on the market, but they don't appreciably accelerate the process. The quickest way to get rid of stumps is to have them ground down with a stump grinder. You can rent the equipment yourself, but it's best to hire a reputable professional tree-care firm to remove the stumps. Their employees will have the know-how to do the job right. Make sure they grind the stumps down at least 6 inches below grade so you can successfully grow grass over the spot. An alternative that is cheaper and will accelerate rotting somewhat is to cover the stumps with a dark tarp and a pile of mulch or soil. Keep it moist. This will be unsightly for a while but will do the job eventually. You could plant some morning glories in the mulch to hide the piles during the summer.
You can also put beds around the stumps (if there aren't already beds there) and plant flowers or shrubs to hide them. You can put flower pots on top of the stumps and plant trailing plants in them as well. A stump can be the focal point of a bed, I did that once and it was nice. In my yard stumps last only a few years before completely falling apart.