What's common now that I am seeing is people ripping out ATX power supplies and getting a molex connector and using those to power arcade machines.
It's actually practical and is becoming an industry standard. I just like the power supplies I use because you can adjust the supply to get the 5V and just use a meter to check the output.
EDIT: Besides the fact that most machines made now are Linux computers running hardware pretty much like a MAME.
You literally can image the harddrive and almost be ready to go where you just need to build a controller for it.
atx supplies have 5v outputs by default though...
could even wire an on/off switch (if it doesn't have one) by putting a switch between the "green" and a ground. (it's usually the green wire but I'm sure a lot of psu's aren't standard).
also good for if one wants to test car/boat electronics indoors without buying an expensive benchtop supply, you can just use a computer atx psu. Just have to make sure you're not exceeding capacity etc.