Don't throw out those used coffee grounds: Your plants need 'em.
Composting is a low-maintenance way to minimize personal kitchen waste, create cheap, nutrient-rich fertilizer, and contribute to the health and productivity of a personal or community garden. Compost is a great gift for plants: It naturally helps replenish the nutrients drawn out of the soil by their roots. Good thing, then, that the same stuff we use to perk ourselves up in the morning can do the same for soil.
Used coffee grounds are rich in nitrogen, which is one of the three main nutrient components—along with potassium and phosphorus—in any successful fertilizer. (For you soil geeks, here's why: Nitrogen allows plants to convert sunlight into energy; phosphorus helps that energy get transmitted throughout the plant through its root system and cells; potassium helps the plant retain moisture, which aids photosynthesis.) When managing the "ingredients" in a compost pile, it's important to have proper balance: Nitrogen-rich material (greens) needs to be appropriately tempered by carbon-rich material (browns).
Coffee grounds are considered "green" matter, or nitrogen-supplying to the compost system. They help create and maintain heat inside the pile by giving bacteria a hospitable growth environment; they also can help manage the pile's overall moisture content. When paired with "browns" like leaves, twigs, even coffee filters, coffee is the perfect catalyst for healthy decomposition, which can speed up the composting process and give you better fertilizer faster. Worms love the stuff: They'll munch your day-old coffee happily, turning it into black gold by digesting it and producing nutrient-rich castings. (Just don't spread the grounds themselves straight on your trees and tulips: They can cause more harm than good if they haven't been fully composted yet.)
If you don't brew coffee at home but do have plants to tend to, you can probably source some grounds through a local café: Many shops offer bags of used-but-compost-clean grounds and paper filters for customers and other garden nuts. It reduces their waste, while helping your tomato plants thrive. (Feel free to tip your baristas in produce come harvest time; money isn't the only kind of green we appreciate.)
Side note: Those "compostable" paper cups that you see more and more at coffee shops are also appropriate "browns," but they're only eco-friendly if you actually compost them. Throwing them away means they will likely end up in a landfill, and the anaerobic environment at the dump stalls decomposition, so even corn cups take eons to break down.
Once your compost is completely broken down into a dense, healthy humus, you can use it to fortify sad-looking tree pits in your neighborhood, plant a window-box herb garden, or donate to a local community or school garden.
Who says you should cut back on caffeine? You're actually doing the world a favor! So keep brewing, and start composting.