Free flat sticks "Painter-sticks"
I don't know how it is in other countries than Denmark regarding this. But in Denmark you can just walk into any shop / supermarket that sells paint, and grab as many free sticks as you want, for free.
The sticks have ads on them, for the company selling / producing the paint, and they are meant to be used to stir the paint before use.
Of some reason it is always really high quality hard wood used, and the sizes (apart from length that varies) are always in matching standards. There are some 3-4 different types, and they match extremely precisely even across different brands.
I am sure there is some interesting history behind this.. But all I care about is the fact that I can get free hard-wood sticks in standard sizes, any time I want.
This is why many of my robot projects are done with these - together with a melt-glue gun, and double adhesive tape (the foamy kind) you can build anything with these.
Of course you can also drill in them, and mount with screws, cut them, layer them to form cubes..
Fast, light, solid, durable, versatile and free. (apart from glue and tape)