Many computer accessories do not have any use within the computer system itself and are simply secondary items used in either the operation or maintenance of the machine. An extremely good example of a non-peripheral accessory would be a mouse mat. Although a mouse mat is not critical to the system it is deemed to be of value due to the increased usability of the mouse that it will facilitate. As such it is an accessory but not a peripheral.
Other accessories might include such things as a cleaning key, a set of tools for repairing the computer, or a special cleaning cloth designed to cause less static build up of the screen and cut down on the volume of dust particles being attracted too it.
USB devices and accessories are also very popular, there are literally thousands of different products out there you can simpley plug into your USB port and awy you go. Items such as simple desktop lights to the Japanese's crazy USB noodle strainer. Here is a short list of other cool USB accessories;
Non-peripheral accessories can often make perfect gifts for the computer enthusiast; they are often inexpensive and are almost always useful. Due to the generic nature of something like a mouse mat there is little possibility of a gift buyer making a bad choice in gifts. Some companies actually manufacture gift sets that contain several popular accessories such as the fore mentioned mouse mat, a screen cleaner, a copy holder (used to hold a sheet of paper at eye level whilst copy typing) and an anti-glare filter.