A lab test analysis showed that: the grounds provide generous amounts of phosphorus, potassium, magnesium, and copper. "Use of coffee grounds in amending mineral soils up to 35 percent by volume coffee grounds will improve soil structure... Use of the coffee grounds at the specified incorporation rates (rototilled into a 6- to 8-inch depth) will substantially improve availabilities of phosphorus, potassium, magnesium and copper and will probably negate the need for chemical sources of these plant essential elements."
They also release nitrogen into the soil as they degrade.
Nitrogen: 2.28 percent
Phosphorus: 0.06 percent
Potassium: 0.6 percent

Starbucks will give you like 50-100lbs of coffee grounds at a time to fertilize your garden with, with or without composting.

Which plants are best to fertilize with coffee grounds.? - Yahoo! Answers