Nikon are well known for high-priced accessories. I'm a sucker for them.
My favourite is the MR-3 - it screws into the Nikon 3-pin socket on certain film cameras to provide a second shutter button as well as use of a cable release.
I can't remember how much it cost me, but it seemed a lot at the time. I now have two.
What's your favourite Nikon accessory?
Edit: sorry about the link - I never learn! Replaced with an image.