When I make a potting soil mix, I add the free grounds from Starbucks. Plants like the nitrogen in coffee grounds, not the caffeine. Grounds are about 2% nitrogen by volume. They are pretty close to a neutral ph, as well. I add a few cups to my large tomato planters outside, and about a cup to smaller planters. Make sure not to glob big piles of it anywhere, though. If it isn't spread out and dug in, it'll grow mold.
Btw, it makes great worm food if you decide to start a worm farm. I add my coffee pods from our machine every day, and they love it.