One thing I have a lot of, and that many people I know have a lot of, is books. Somehow we have shelves and shelves of books all over the house, and we don't re-read most of them. Most were read once long ago. So surely I can whittle us down to just say 500 books and get rid of the rest.
So first I look on Amazon to see if it's worth something. If so, I list it there on the used market. It's free to list and they take a commission if it sells.
If it's on Amazon for just a penny, then I go to PaperBackBookSwap and list it there. When I send it off, I get a credit and can then let a friend get herself a free book. So it helps multiple people all at once.
I also routinely send notes out to my friends of my unwanted books, and they take some of them too.
I used to have PILES of book all over the house because I get deluged with free review copies. Now that I've made a serious effort, I now have much smaller piles and they are clearly labeled that they're listed on Amazon, listed on PPBS, or are waiting to be reviewed by me.