Can you use what's left of a lemon after zesting and juicing?

You can also use the left overs to clean out stainless steel pots & pans. My mom throws lemons into her pressure cooker with some water, lets it go for a few whistles, and then is able to clean the inside of the pot easily if there any stubborn spots. I'm sure you could the same with boiling water in regular pots/pans if you dont have a pressure cooker!

I don't have much use for the pith, but I save the the membrane (and pips) in freezer bags for when I make jams and jellies. There's a tremendous amount of pectin in the membrane and pips. When I'm ready to use them, I place them in a cheesecloth and tie them up so they can be removed easily then cook them with the fruit.

i clean my copper pots with lemon and salt. chemistry in action.

I put them in a mason jar and fill with vinegar. Let it steep for as long as needed (as little as two weeks or even two months). The I strain into a spray bottle and use it all over the house for a natural cleaner. My husband even loves it in the garage for greasy stuff.

I just add a little water and put it in the microwave on HIGH to clean and freshen the microwave. It gets rid of bad odors inside the microwave. To make this effective,clean the inside of the microwave with a damp paper towel.

I cut the remainder of lemon in pieces and put them in a ziplock. Once a day I will toss a piece in the disposal to keep it smelling fresh. Usually there is about a weeks worth of lemon pieces.

I often use lemon (for juice, salad dressing, marinade) if I've zested OR juiced it, because there are still good flavor elements left.
But the (bitter) pith and (fibrous) pulp doesn't have much left in it.
You could buzz it up with some sugar to make lemonade, or just add it to compost.