The Money Hacker is a combo weapon that can be made by Chuck and Frank in Fortune City. It is created by combining a computer case with a flashlight. It can be used to shock enemies and hack ATMs and slot machines to make them eject more cash than normally. ($2,500 per ATM if smashed open, $10,000 if the hacker is used or $5000 per ATM if smashed and $20,000 if the money hacker is used in Off the Record). The hacker itself lasts for about 6 ATMs if used correctly. At the maintenance room beside Finders Peepers are both the flashlight and computer case available.
"Hacking" is a popular term for manipulating computer data. It's likely that the device was named for this, noting its ability to "hack" ATMs.
The easiest and quickest method of making the Hacker is to enter the Palisades Mall, through the shortcut from Hot Excitorama in the Silver Strip, or, failing that, the Atlantica Casino. The service hallway there contains a Maintenance Room with both a flashlight, and a computer case. (To the left/western side if you enter from the Casino.)
In Dead Rising 2: Off the Record the Hacker is called the Money Hacker, and can be made rather easily using components near the first workbench Frank encounters.
The Looters in OTR can be found wielding Money Hackers.