I usually donate mine, as intact sets, to thrift stores.   They can get a buck or so for a used set, or they can use them to restring a guitar someone has donated.
(You'd be surprised how often I see a guitar at a thrift shop that is missing strings or has strings so bad you couldn't possibly tell how the guitar would sound.   If it has a full set of semi-decent strings, they can get a better price for the guitar. Or, if the guitar still plays and sounds like crap even with those strings, there's less chance some sucker will pay too much for it.) :-)

I also like the idea of donating the used strings to organizations that will give them to beginning students or people are really strapped for $$.

Of course, used guitar strings are also handy for strangling banjo players and accordionists. ;-D

Genie