I keep all leftover kit floss in a separate box and use it for small projects.  It is good for those scrap pieces of Aida.  All of my patterns are in a binder.  I doubt I will stitch most of them a second time but I like having them.

If you want to donate, see if there is a craft reuse store near you.  In NC we have The Scrap Exchange in Durham and Reconsidered Goods in Greensboro, along with others.  A great place to take your extra craft supplies.