The S9V11MA switching step-up/step-down regulator efficiently produces a finely adjustable output between 2.5 V and 9 V whether it is higher or lower than the input voltage, which can range from 2 V to 16 V.  Pololu 2869 

Power supply based on DC Step Down converter with adjustable output voltage from 0 to 16.5 V and current efficiency of 3 A. The set includes LCD display and transparent case

VRM: Voltage Regulator Module (Rev. B) is a step-down converter based on the Texas Instruments TPS54620 chipset. Input voltage in the range 5 ... 15 V, output voltage 3.3 V or 5 V. Digilent 210-156

This synchronous switching step-down (or buck) regulator takes an input voltage of up to 40 V and efficiently reduces it to 3.3 V with an available output current of around 15 A. Pololu 2880

Pololu 2832

Step-Down DC-DC converter module based on the D24V150F7 chip. Input voltage 9..40V, output voltage 7.5V (max 15A). Pololu 2883

The MyoWare Power Shield is designed to take two standard CR2032 coin cell batteries to power the MyoWare Muscle Sensor. We have made this board in a way that the coin cell batteries are connected in parallel for extended capacity at a nominal 3.0V. DEV-13684

This small synchronous switching step-down (or buck) regulator takes an input voltage of up to 36 V and efficiently reduces it to 7.5 V. The board measures only 0.7″ × 0.7″ yet delivers a typical continuous output current of up to 2.4 A and features reverse voltage protection.

The very small step-down 6 V, 2.5 A synchronous inverter can be supplied with voltage up to 36 V, dimensions: 17.8 mm x 17.8 mm. Pololu 2859

The compact (0.4″ × 0.5″) D24V5F2 synchronous buck voltage regulator takes input voltages between 3 V and 36 V and efficiently reduces them to 2.5 V while allowing for a maximum output current of 500 mA. This regulator offers typical efficiencies between 80% and 90%. The pins have a 0.1″ spacing, making this board compatible with standard solderless breadboards and perfboards.

The compact (0.4″ × 0.5″) D24V5F9 synchronous buck voltage regulator takes an input voltage of up to 36 V and efficiently reduces it to 9 V while allowing for a maximum output current of 500 mA. Pololu 2845

This synchronous switching step-down (or buck) regulator takes an input voltage of up to 38 V and efficiently reduces it to 5 V with an available output current of around 9 A.

This small synchronous switching step-down (or buck) regulator takes an input voltage of up to 38 V and efficiently reduces it to 9 V. The board measures only 0.7″ × 0.7″, but it allows a typical continuous output current of up to 2.5 A. Typical efficiencies of 85% to 95% make this regulator well suited for powering moderate loads like sensors or small motors.

Pololu 2834

Step Down converter module with an output voltage of 5 V and a maximum current of 3 A. The board is equipped with two USB connectors

Pololu 5V Step-Up/Step-Down Voltage Regulator S18V20F5

Step-Up/Step-Down converter with an output voltage of 5 V, an input voltage from 3 to 15 V and a maximum current of up to 600 mA

Step-Down converter with adjustable output voltage from 1.25 to 36 V, input voltage from 4 to 38 V and maximum current up to 5 A. Board equipped with a voltmeter with display

Step-Up/Step-Down converter with an output voltage of 3.3 V, an input voltage from 3 to 15 V and a maximum current of up to 600 mA