If you went to any country of the Euro zone, before it changed currency, you probably have coins in the old national currency lying around. As those coins are not legal tender anymore, they are gathering dust and you don't have much use for them. Unless you are a collector of course or are in need of worthless round pieces of metal for whatever reason. If you don't have a use for them, I suggest you give them to Oxfam. They will take them and change them or sell them as collectible items, generating revenue to support their charity work. The simplest way to do it is to go to your local Oxfam shop. If you don't have one, you can send them to their Stamps & Coins Unit. Here is the address for the UK:Stamps & Coins Unit
26, Murdock Road
Bicester OX26 4RF
I dropped mines at my local shop at the week-end and that's one less bag of useless stuff gathering dust on my shelves.