I use the new tubes for refilling empty pans. If the semi-dry ones are still squeezable, they can work even better for refilling, because they don't drip liquid. You can cut open the dried out tubes with a sharp knife. The pigment is often tar-like in consistency and can usually be scraped out with a palette knife. A little water pressed in with an old spoon is usually enough to reactivate them. If you're handling toxic pigments with your fingers, remember to wear gloves.
My jar of pans is a graveyard of colors I've dumped from sets because I wasn't interested in them. Sometimes I change my mind and give those refusés another try. Every six months or so, I change my palette selection to keep myself off balance.
If I'm sure I don't want an old pan color, I pry out the color so that I can fill the empty pan with a new tube color. After refilling it, I put it on the sill of a sunny window and let it dry out for a week or two. If it cracks after drying, just fill in the cracks with more liquid color and let it dry again (thanks Jobot).
To cure the pigment from drying too crumbly, add a little gum arabic to it. Gum arabic is the binder or gluey stuff that holds watercolor together. You can get it in powdered form and it's non-toxic. It's also used for gluing cigars and making royal icing more shiny. You can even use gum arabic to make your own watercolors out of your dry eye shadow or dry pigments. (thanks, DKVision and Jobot).
Check out my video:Watercolor in the Wild by James Gurney
Big post about materials