The Step-Down D24V6F12 Buck Voltage Regulator module gives the output voltage of 12V with a wide input voltage range of 15-42V and a maximum output current of 600 mA. Pololu 2109

These stackable 0.1" (2.54 mm) female headers are just the right height for connecting  Arduino Uno, Arduino Mega 2560, Arduino Leonardo. The set includes two 1×6 female headers, two 1×8 female headers, and one 1×10 female header, so it has everything necessary for both old- and new-style shields. Pololu 1035

The Step-Up / Step-Down S18V20F12 inverter module gives the output voltage of 12V with a wide input voltage range of 3 ... 30V and a maximum output current of 2A. Polol 2577

This NEMA 17-size hybrid bipolar stepping motor has an integrated 28 cm (11″) threaded rod as its output shaft, turning it into a linear actuator capable of precision open-loop positioning. The stepper motor has a 1.8° step angle (200 steps/revolution) and each phase draws 1.7 A at 2.8 V, allowing for a holding torque of 3.7 kg-cm (51 oz-in). Pololu 2268

This tiny logic level shifter features four bi-directional channels, allowing for safe and easy communication between devices operating at different logic levels. Pololu - 2595

Linear potentiometer with force sensor. It has an analog output, it allows you to detect where and with what force the sensor strip has been pressed. Pololu 2729

Pololu - 2129

Pololu 2258

Great for learning the basics of gears and linkages, this Tamiya kit contains all the parts necessary to make a remote controlled crawler that picks up ping pong balls. Pololu - 2389

66 channel GPS receiver module LS20031. It communicates via the UART TTL interface with a refresh rate of up to 10 Hz. Pololu 2138

Pololu 2450

A set containing a set of connectors, jumpers and buzzer for the A-Star platform. Polol 3110