Although a bench PSU is useful, sometimes I prefer testing with something more portable. This ultra portable PSU has a footprint smaller than a credit card. It is perfect for travel. (Pen shown in image below for scale). I can charge it up beforehand (or take a USB cable with me) within a couple of hours, and be confident it will work for tens of hours depending on what is being powered. The wires pack inside a sliding door.

It is very low cost, and very quick to build; it uses mainly off-the-shelf hardware. The 1.4AH LiPo battery (6 euro) and charger circuit (4 euro) is from Olimex. The OKW case is from Farnell and was a bit expensive; I already had it, otherwise I would have used a far cheaper case, maybe even cardboard, since this is not intended to be pretty. The 5V switching converter (to boost the LiPo voltage) was from eBay (less than $6). I also decided to have a lower power 3.3V output too; this is based on a single LP2950ACZ-3.3 and a 10uF ceramic capacitor. Everything was just held down with epoxy resin glue.

The three wires (+5V, +3.3V and 0V) are long enough to be useful, and there is a notch so that they can be left extended or they can be fully packed inside the slide compartment.

There are some 2mm dia holes to indicate USB power, charging, and charge complete (these LEDs were part of the USB charger circuit):

There is a small slide switch for power; the LiPo cell is permanently connected to the charger circuit (it is prevented from discharging by the charger circuit) and the switch just applies the LiPo battery output to the 5V DC-DC converter and the 3.3V linear regulator. You can see the switch wiring in the detailed image earlier.

Here it is powering some hardware:

Note: After I constructed it, I saw on the Internet that PopSci had a similar idea many years ago, see here for construction details.