A fidget spinner is a simple and basic handheld toy meant for mindless fidgeting. These toys are one of those "I don't get it" things, until you play with one. Fidgeting is actually very helpful for people with ADD/ADHD and even anxiety, but you don't have to have a disorder to appreciate them.
There's been a sudden explosion of fidget spinner designs on sites like Thingiverse.com. Most of them use a standard 608ZZ ball bearing to spin freely. They can be purchased online and cost anywhere from $10 – $20, but you could easily 3D print and make your own for a few cents! The recent trend of fabricating your own seems to vibe nicely with the maker community and even more so with 3D printing.
A ball bearing is a type of rolling-element bearing that uses balls to maintain the separation between the bearing races. The purpose of a ball bearing is to reduce rotational friction and support radial and axial loads. It achieves this by using at least two races to contain the balls and transmit the loads through the balls.