If you are at all like me and have been doing photography for awhile you have a drawer or box of old "accesories" that did not prove to be as useful as they seemed they might be when they caught your eye and pocketbook. For the digital SLR user I recently got a Hoodman HoodLoupe that is designed to magnify and shroud the LCD panel on the back of the camera. If you are out in daylight making menu changes or reviewing just shot images it's often hard to see the LCD image on-screen clearly. The HoodLoupe is an ideal way to overcome that limitation in brightly lit environments. It is one accessory that I am sure will stay in my carrying bag permanently and will get lots of use.
For me it is the accessories for older cameras that have accumilated over the years since they can not be used with my newest digital camera.
The other item that I never seem to use is my 20GB wolverine digital storage. Got it before I had a laptop took it on some trips, never used it until we had a hurricane pass through here and lost power for 3 days. Never used it since but it is in my backup camera bag with some other items.