First, look at these two pictures:While looking at both photos, cross your eyes. With your eyes crossed, you'll see three photos instead of two. Try to focus on the middle one. It takes a little practice, but when you get it, WOW. 3D!
This technique is called "cross-viewing" and is similar to the old-fashioned stereoscopes you see from Victorian times. The photos are by yowayowa camera woman, aka "the levitating girl." Her photos are gorgeous and oddly mesmerizing even without the 3D affect, so you'll want to see her other work, too. (And thanks to Mary B. for the link!)
Her site doesn't have a search bar to find just the 3D images, but I tracked down a few more for you here, here, and here.
Oh, and if you can't get your eyes to cross properly, Mental Floss just posted 12 animated stereoscopes which give the same general 3D effect without all the eye strain. :)