One popular and traditional hacker enclosure has been the classic metal Altoids peppermint can - to the extent that some products and prototyping PCBs are shaped to precisely fit one.
(Image from Adafruit)
For more examples, see a toy, an amplifier and another, a sound generator, a USB charger, one could go on all day.
A fringe benefit of using a metal can like the Altoids, is EMI / RFI reduction, both emission from poorly designed circuits, and from the outside into the circuit.
For smaller circuits, round metal shoe polish cans are popular, and again qualify as "EMI-Safe Device Enclosure".
(from Wikipedia)
Another "hacker's standard" that has been around for decades is the wooden cigar box. They're sometimes found at garage sales or the scrapyard, in a variety of sizes and designs. My favorite are the ones with a double hinge, and a little metal latch in the front. Back in college, I built myself a bench power supply in a big cigar box, that is still around somewhere.
The fringe benefit of wooden cigar boxes is protection from electrical accidents when working with main line power input to your device.
A third standard go-to option in cases where robustness is not a concern, is the small Pringles or other potato crisps cardboard can. They're especially convenient for cutting holes in, for sockets and connectors. The 2 to 3 inch height and diameter make such boxes useful for circuits with a transformer in them, such as non-switched (good old) power supplies.
Finally, plastic enclosures aren't necessarily expensive: You can sometimes pick up assorted sizes in lots of 5 or 10 from eBay for under 1 US$ a box, and manufacturers offer a variety of standard enclosures starting in the $3 range, probably cheaper if you search around.
