I'm experimenting with waste coffee grounds for growing vegetables from seed. Not sure how it will go, but the benefits are that they hold water well, are finer that the commercial pine-bark potting mix I've been using, fairly sterile (from fungus and bacteria), hopefully nutritious since they're crushed seeds, plentiful, free of charge and smell quite pleasant. :)

Downside may be that the pH is too low. My pH test kit say the grounds are about pH 5 and the kit notes recommend 5.5 - 6.5 for potting mixes, so not too far off. Some quick experimenting seemed to suggest that adding dolomite limestone at 1:8 gave me a pH of about 6. Seems like quite a lot of dolomite, so I might have to test again later and see if the pH settles higher as it breaks down.