A tele-converter is a budget accessory that will increase your photography options. The most common one is a 2x converter that doubles the focal length of your lens. Your 50mm lens becomes 100mm, your 70-300mm zoom becomes 140-600mm.
Vivitar made a number of converters through the 70s and 80s.
Teleconverters are ideal for photography such as sports and wildlife.
Advantages:
- increased magnification
- light weight
- cheap
Disadvantages:
- light loss
- quality degraded
Several teleconverters converters for sale here