The Simple Arduino published earlier can easily be outfitted with a 3.3 Volt outlet, as there is space enough on the stripboard
Adding 3.3 Volts is easily done with a 3.3 V regulator for which I have chosen the lowdrop TS2950C33. This regulator can deliver 150 mA which should be enough for most applications.
The circuit is quite straight forward, just use the input of the 5 Volt regulated power supply of the board as a feed for the TS2950. If you pick another 3.3 Volt regulator, make sure that 5V input is enough, otherwise you may need to feed it from the unregulated power line coming from Diode D1. The capacitor in the 3.3 V line does not need to be that big: 1uF even 220nF is probably enough already, but I just happened to have a 10uF.
It is very easy to find space in the setup as originally proposed by Iñigo Zuluaga, as shown here:
It requires only 2 extra wires, a TS2950C33, a capacitor and of course a one pin larger connector and it nicely fills the ‘empty space in the upper right corner.
The Stripboard looks like this: It requires cutting a strip and running 2 wires. (indicated in red). If you wish, you could extend the header with even 1 more hole to not only include the 3.3 V pin, but run an extra Ground right next to it. The ground is already running on that strip