I do.
Donkey Kong (Apple II version) - spring level:
I would stand under the falling springs until they pounded me through the floor. Then I would fall through the top of the level and rescue the girl.
Diablo (original):
During online multiplayer, someone taught me this trick where if you drop an item and pick it up quickly you can duplicate it.
Command & Conquer (original):
While playing the GDI campaign, I discovered that enemies do not attack walls and just target the units and buildings behind them. During the campaign, I built sandbag barriers to keep enemies away from my base, and away from the Tiberium as well.
Maplestory:
One time the clipping was broken (monsters wouldn't hit me) so I used the opportunity to complete a jump quest where moving balls would normally knock you off.
So tell me; what bugs have you exploited during your entire gaming career?
Edit:
Age of Empires II (original non-HD version):
After busting through the gates of the Great Wall of China and building my own, I discovered that the enemy didn't attack my gates. I would use gates to wall off my enemy at narrow points.
A truly prophetic sig...