maybe they search for the internet for free stuffs and add them on their websites. Or possible, they have created a mailing list, where all persons who want to give something for free subscribed, and when someone wants to give something for free, they just send an message to the owner of the website
yah, but I'm assuming it is automatic since it will be very time consuming to search for deals and insert them manually. Also, I noticed most of the free stuff are submitted by 2 users, so it is likely to be the site owners themselves.