I have been given bags of fabric also, and this is what I do!! I keep the cotton, and donate the polyesters to a craft store or your local Salvation Army or Goodwill. There are people who make things out of polyester. I keep anything that starts with the letter COTTON! I don't know for sure when I will use it, but someday I will. I have also said to the person gifting me "I only use cotton, but I can donate this fabric if you are sure you don't need it for anything. There are a lot of people that sew with polyester." You're killing two birds with one stone here. And the person will appreciate it also - because she will know it is going to a good cause.
Happy Cutting! Edie