I once had to take an X-Acto knife to the backplane of something (VAX or PDP-11 or vice-versa to get a card for the "wrong" one to work).
Another time, I needed 240V, but the office only had 110V, so I made a "two headed" extension cord - two normal plugs, and one 240V twist-lock socket.
Then All I had to do was find two outlets on opposite phases and I was in business. This item would probably NOT get UL approval.
Not really a hack; but as a software guy there was one problem driving me nuts, to the point that I had to figure out how to use the logic probe to prove to the hardware guys that the circuit board they layed out had flipped the upper and lower 8-bits of a 16-bit buss... I fixed it in software (until they did a new spin of the board and fixed it, as it was just a prototype).
That was back in days when the CAD program would take days to route a board, and PCs were expensive. We made a moderately priced PC into an expensive (faster) one by changing out the clock crystal - back in the day when "overclocking" was a hardware hack.
Best modern hacks? Root your Android phone and put Cyanogen on it, for crying out loud, stop whining about bloatware!
Also, not too long ago, my kid complained that the "windshield washer fluid" warning light was on all the time (older Camry), but there was plenty of fluid and it worked fine. So, rather than "fix" it, I figured out which wire needed to be snipped... problem solved. (Now you have to "manually" inspect the fluid level of the washer fluid - oh the horror.)