Optical Slave Triggers – a piece of accessory which is tremendously useful if you work with strobes/speedlights. These are tiny in size but can help you accomplish wonderful photographs. It simply is a light sensitive sensor within which senses the light and triggers the circuit which then ultimately triggers the attached strobe or speedlight.
A good flashgun is an indispensable accessory for any photographer. If you don’t have one, you should definitely get one. This is one of the accessory which I highly recommend would improve your photography. However, getting one is not a walk in the park as the market is flooded with hundreds of manufacturers offering tons of flashgun. Separating chalk from cheese is the order of the day. Further, there is no single flashgun which suits all. It all depends on what you would be using & how much budget you can allocate.