Books.

I work at a library which allows me, not only a lot of books to borrow, but first hand picks at books we must get rid of. I know this may come as a shock to avid library patrons, but we can't possibly keep all books that are ordered for us and with time, some of those books are removed from our shelves and sent away (to where, I do not ask). If I'm lucky, I get to snag a few interesting titles for my own library at home.
As a reader, you probably have a large personal library of your favorite books. I've found that not only do I have a wealth of my favorites but also a lot of books I've acquired that I hope to read "someday".
I keep these books to inspire me, as helpful sources for research and to continue my personal world of gluttony I've established since my early childhood.
As a writer, you must keep reading, no matter what. This helps you stay fresh, gives you new ideas, and continually reminds you of appropriate grammar and spelling. By reading, you make yourself into a better writer; more informed about the craft, more informed about history/science/etc., and more inspired to try new styles or to venture into new worlds.
Be you a reader or a writer, I say keep your library full and your mind open. Even if you don't want to be a glutton like me, you can always get a free library card and utilize their shelves as your personal collection.
And, it has been suggested, that if you write fantasy, read (at least on occasion) outside of that genre. I like to read historical fiction and non-fiction, I don't know if it helps with my writing of fantasy, but I know it can't hurt.