Digital to Analog Converters. These take serial or parallel data and turn them into an analog signal. Of course you can use a R2R resistor ladder but these are compact and can use fewer pins

Image	Part # & Datasheet	Description	Distributor	Eagle library part
	MCP4921	The cheapest 12-bit thru-hole DAC we found Input via SPI	Mouser Digikey	Adafruit:MCP4921

Analog to Digital Converters. These take analog in and turn it into serial or parallel data.

Image	Part # & Datasheet	Description	Distributor	Eagle library part
	MCP3008	The cheapest 10-bit 8-channel thru-hole ADC we found Output via SPI	Mouser Digikey Adafruit