I've just found out about this: Don't throw your old bras away - Donate them to charity with the BreastTalk.co.uk Bra Appeal. This is what they say:
We're asking for all your old bras as long as they are in good condition and straps and clasps are in good working order. Send them to us and we can distribute them to charities throughout the UK and overseas.
There are many women across the world who for some reason don't have a bra. They may be homeless in the UK or living in a country where the price of a bra is more than a meal for their family. The BreastTalk.co.uk Bra Appeal is trying to help these women by collecting and distributing your unwanted bras to charities in the UK and abroad. http://www.breasttalk.co.uk/bra-appeal/
What a great reason to clear out your undie drawer!