JY-MCU 8X Red Light Digital Tube + 8X Key + 8X Double Color LED Module

Pros: - Very useful to get numeric and/or state output and to provide input to a sketch into an Arduino using only 3 pins.
- Rather cheap for all the functionality provided.
- The little ribbon cable used to chain several modules can be used to connect the Arduino, when soldering a adequate outlet on a prototype shield.

Cons: - Build quality is rather average.
- Finding datasheet about the chip used is very difficult.

Other: This is obviously a must have for any Arduino hobbyist.
It can either be used for its own sake, just for fun, or to provide a good way to display some internal values when building something bigger. For example, buttons can be programmed to select a given value to display.

Pros: This is a very low cost module for what is on it. With all the buttons, dual colour LEDs and full 8 digit display, an awful lot of information can be conveyed. I used it for a ping time tracker, using the LEDs as a bar graph and the actual results on the LED digits. These can be chained to multiple units using the included cables.

Cons: None that I've run into, they respond quickly and operate well.

Other: Chaining three together to an Arduino makes for a nice little stock ticker.