There's not a whole lot of coffee drinking that goes on over here at the Gardens (it only takes one cup to overcaffeinate me, and it ain't pretty), but when we do brew a pot I save those grounds for my hydrangea.
A bunch of acid loving plants really enjoy their joe when you work some used grounds into the soil - hydrangea, blueberries, roses, rhododendron and azaleas. I hate azaleas. And I have three awful scrawny ones left behind by the previous owner. Think it will help if I give them a little caffeine?