Kaiko Spikey fidget has been an instant favourite!
It provides firm and textured sensory feedback when rolled up and down the finger. It is touted to have remedial benefit, stimulating acupressure points & assisting with circulation. These are stretchy but firm.
 
It is particularly good for those that pick and bite as the 'bitey' sensation stimulates in a similar way without the harm. 

Some ways to use the spikey...
Roll up & down the finger. It can be rolled using the same hand or alternate hand.
Rub between two palms
Hold in one or two hands to fidget
Place on finger & twist around using thumb (similar to a 'Chinese burn') applying varying degrees of pressure-dependent on preference. This technique has been particularly helpful for some in minimising self-harm. It can provide the 'hurt' without harm to the skin. Of course, the spikey fidget does not replace accessing professional help. It is a useful tool for the kit-bag.
Place on one finger & squeeze adjacent fingers into it.
 
Finger Spikey can help with self regulation, calming, focus, concentration, tactile awareness and as a substitution for unhealthy/unhelpful behaviours.