Useful Clutter

We live near a lot of family and because of that we are frequently given their old toys, clothes and furniture. I am always extremely grateful for these offerings. I haven't had to buy a lot of clothes or toys for my kids and we've saved a lot on furniture as well. The problem for me is that I can find a use for anything with shelves or drawers. I just have too much of it. More than I can fill. But when you hold onto it, eventually it fills up. Bare surfaces always collect clutter the way bare dirt will always grow weeds.
And yet I can't find it in myself to get rid of it. I love to rearrange and reorganize rooms in my home. So the bookshelf I'm not using this month may be used next month. I rationalize keeping it because it might be useful someday. It might. Meanwhile it's taking up valuable square footage in my home which makes me cranky.
My 2 kids share a room right now and the other bedroom is used as a playroom. It also houses all the extra "stuff." I spent the weekend putting old toys and things into the donation box and was left with 2 empty small bookshelves, a file cabinet, and a dresser. The dresser is now storing tools in my shed because I can't bear to part with such a useful piece of furniture. But that file cabinet has been empty for a month and I just now placed it for sale. Furniture really is my clutter nemesis. The bookshelves are sitting in the playroom slowly collecting toys and books as my brain tries to justify a space for them.
What I'm trying to remember is that I value having less stuff. Even if it's valuable or potentially useful, it's not worth keeping if it doesn't bring peace to my home, right?
What do you have a hard time getting rid of that you know you should/really want to?