I keep folders on a hard drive for free items, so I can credit the maker later. I've got a folder for where it came from, and then a folder with the artist's name. For example, if I had downloaded something of my own from here, I'd put it in FreeStuff/Renderosity/CaptainJack1. I always keep the original archive, too, in case I need it, so all of the information about it is there. It makes it a lot easier to credit the artist I got it from later, or to contact that person if I have a question about it.