One of the most indispensable items to me are the pheromone diffusers. They mimic the pheromones that a mother kitten uses to help calm her young kittens, It is both drug free and natural. From experience I can say they do help with things like helping a kitten to acclimate to a new home more quickly. They can also help with reducing behavioral problems such as inappropriate scratching and fighting with other cats.
Every kitten goes home with one of the collars but setting up the diffusers a week or two before your new kitten comes home can help make the transition easier for them.
One of my favorite training aids is the SSScat. It is really good for teaching the kitties where they can and can't go. Also very useful for training them to not try and go outside on their own. One of the really good things with it is that because it triggers without you being around it shows them that they don't only have to follow the rules when you are around. It also means that when they try to go somewhere they shouldn't and it triggers you aren't the bad guy which helps to keep your relationship with your kitten strong.
All products listed here are things we have used to great success and are not sponsored or paid advertisements.