I recently discovered that a friend was coming in from a neighboring town and collecting coffee grounds from behind a café that I sometimes write from. I got all territorial with him and suggested these were for 'locals only'. I was joking, of course, but it is true to say that spent coffee grounds are a sought after commodity around here.
As Jasmin has noted before, adding tea and coffee to your compost can be a great way to use a waste resource, and they are included in Nicole's list of instant homemade fertilizers. But beyond simply dropping your used filter in the compost pail - why not look for industrial sized quantities? The occasional bucket or two of coffee grounds added to a large compost heap can get it started like almost nothing else (except maybe chicken or horse poop). Many coffee shops leave buckets and buckets of grounds outside each day which are rapidly collected by farmers and gardeners alike - especially those of us who are growing acid loving plants like blueberries. And if your local espresso bars don't yet offer grounds for composting, why not ask them to start doing so?
Of course when we are talking about large quantities, you need to make sure that grounds are well and truly composted before you spread them around your plants. If you don't think you have enough material to create a hot compost heap—remembering of course that adding cardboard to compost can be a great way to bulk it out—then you could also try growing oyster mushrooms in coffee grounds. Once the shrooms are done doing their thing, the coffee should be well broken down and ready to add to your soil.