Back in the day, I used to hack/mod just about anything that I could. Single player only of course.
Diablo 2 - I made a mod for my own personal use which raised the level cap to 125, added a new class of items above Elite (so there was Normal, Exceptional, Elite and Super Elite), added new uniques and set items, allowed belts, gloves and boots to be socketed, added new champion monsters, removed the experience penalty at higher levels, smoothed the experience curve so that your levelling rate did not drop off so rapidly, etc etc. Was quite big.
Baldurs Gate 2 - in addition to using the available mods, I also once modded the game for my own amusement. I made two characters that were a fighter/mage/thief and fighter/mage/cleric. I gave them special powers either by editing the character class or giving them each a set of custom chain mail. Probably both. I think I made it so that they could both use the Inquisitor's dispel magic and they gained more uses of it the same as he would. They also had powers belonging to other classes, like the Kensai's bonus to hit and damage and the wizard slayers magic resistance. But none of the penalties of course.
The biggest thing that I did to them was give them permanent -4 casting time and permanent improved alacrity. So even at a low level, I could cast multiple spells per round. It was insane - they were devastating.
I modded GTA Vice City and San Andreas to mess with the physics. I made Lambos weight like 100 tons and able to accelerate really quickly. You pushed the accelerator and it knocked everything out of your way.
I made a new side for C&C Generals: Zero Hour. They were a combination of the other sides with some really awesome stuff. All the best of each, plus the bonuses from research would apply to vehicles from other sides. Obviously, they were tremendously overpowered. I also experimented with new super powers.
I modded C&C Red Alert, Red Alert 2 and Tiberian sun like crazy. But mostly just altering existing stuff.