Excellent digital alternate to old analog potentiometers. Also has built in momentary switch when pressed. Normally you can find it on digital oscilloscopes with continuous rotation in both directions.
This rotary encoder is high quality, with detents(steps feel) and a nice continous rotation. It is panel mountable for placement in a box, or you can plug it into a breadboard (just cut/bend the two mechanical side tabs.). This encoder also has a push-button built into it so you can press onto the knob to close a separate switch. One side has a 3 pin connector (ground and two coding pins) and the other side has two pins for a normally-open switch.
Rotary encoders are useful as rotation sensors or selectors and look similar to potentiometers. However they are not like potentiometers at all, so it's important to realize the difference! These rotary encoders rotate all the way around continuously, and are divided up into 18 'segments'. Each segment has a click-y feeling to it, and each movement clockwise or counter-clockwise causes the two switches to open and close. There is no way to know what the current 'position' is - instead you would use a microcontroller to count how many 'clicks' left or right it has been turned. If you need to detect rotational 'position' a potentiometer would be a better choice.
Details and Datasheet: Alps Part Number: EC11E09244BS (All function details are same except knob type). It has knob type as shown in main product image and not below type.