I've got a large box full of copper fittings (elbows, caps, tees, etc.) used for household plumbing.  Any ideas for something I can do with them?

scrap them for cash

If they are new, sell them to a plumber since scrap copper has gone way down in value.  I'm guessing since you said home use that they are under 1" and most likely 1/2". They can be cold worked into artsy pieces, read up on annealing copper.  Copper is a pain to cast due to its affinity for oxygen = porosity as cast.

Plenty of uses for plumbing fittings, not the least of which is domestic plumbing! Cups for candlesticks, ferrules for chisels or screwdriver handles, they can also be used to make decorative lamps (electrical). I've even seen them incorporated into door handles. A clear laquer coating saves on constant polishing.