For reasons of pride (and, as my grandmother always said, "What if there was a fire and the firemen had to carry you out?"), I guess we all have regular purges of our old well-worn underpants.
I just bought a raft of new undies at Target and, in the spirit of "going green," decided I could use the old ones as painting rags. It makes you feel like you're really doing something for the environment! Since, let's face it, that's one contribution of "old clothes" that the Salvation Army isn't going to thank you for.