I did a quick look over the topic to see if my idea has already been posted yet, and it hasn't, so I'll throw out my idea.
Pushing On
Concept: In a fight, though you may not pull your punches, you won't go all out either. You will instead pace yourself, holding back some strength to keep from tiring yourself out. However, if you want, you can Push On, to hit harder and more solidly. However, this tires you much faster, and if you push yourself too much, you can knock yourself out!
Mechanics: As a free action, you can Push On. This lets you add your character level to attack and damage rolls for a number of rounds equal to your CON modifier (min 1 round). At the end of this duration, you are fatigued. If you push on while fatigued, you receive half your level as a bonus (min 1), and are exhausted when the effect ends. If you are exhausted when you push on, your bonus is equal to one-quarter your level (min 1), and fall unconcious for 10 minutes when the effect ends.
Submitting the content for criticism and editing. You may use this in your 3.5 'patch', RobbyPants.