The above picture shows PX4 Airspeed (in fact its so new this is even a picture of a (proven) prototype), a small, digital airspeed sensor. If features a Measurement Specialties (coincidentally swiss, too) 4525DO sensor, with 1 psi measurement range (roughly up to 100 m/s or 360 km/h or 223 mp/h). Its resolution of 0.84 Pa is quite good, and delivered as 14 bit data from a 24 bit delta-sigma ADC. It also measures temperature to allow to calculate true airspeed from indicated airspeed using the MS5611 static pressure sensor on Pixhawk. As the temperature is not influenced by the heat of nearby processing components, it is much closer to the air temperature than with the previous analog sensor setup. It comes with M3 / 6-32 mounting holes. It is supported on all PX4 autopilot generation boards (and in the PX4 flight stack as well as on APM on PX4). Andrew Tridgell has intensively test flown the sensor and will share his experience soon.
This is just the fanciest peripheral, but maybe not the most popular one: The high-brightness multicolor led featured inside PX4 is also available as external option, and it is even supported by the APM 2.x boards.
Last but not least a peripheral only available for Pixhawk and simple yet convenient: An USB port extender, which allows to mount the micro USB port outside of the fuselage / shell. In contrast to normal USB extenders this one is for DF13 plugs and thus does not require the enormous radius of a normal USB cable in the fuselage.
All these peripherals are intended to improve the ease of use and robustness when working with a PX4 based system, and will be available shortly.