I was reading my favorite garden forum the other night and ran across a thread on coffee grounds. While they proved unsuccessful for germinating seeds several people used them as soil in potted plants.
They also reported success propagating things like roses and other shrub like plants. Branches were cut, lower leaves removed, rooting hormone applied and then potted in used coffee grounds. All people doing experiments reported seedlings did better in coffee than in soil.
Used coffee grounds act as a mild fertilizer ~( 4, 1, 3 ).
The caffeine in coffee is thought to be a neurotoxin to several unpleasant little critters in your soil such as slugs. I found reports stating they both helped and harmed nematodes. Nothing conclusive either way there.
Grounds are acidic so are a great mulch for your acid loving pants.
Coffee is high in nitrogen(~2%) which makes it a good fertilizer for plants indoors and out. Used coffee grounds from your coffee pot filter also contain potassium ( >1%), phosphorous (~.3%) and other trace elements.
You can mix some grounds into the dirt when you repot your house plants. Or you can sprinkle some used grounds on the top of the soil of your house plants. Use them sparingly. The nitrogen is released quickly, not over time, and you do not wish to give your house plants fertilizer burn.
Plants that prefer acidic environments especially like to have some coffee grounds added. Also foliage plants ( non-flowering) do very well with coffee grounds.
It is better to let them dry out before adding them to your plants. If they are put on too thick while wet they can mold.