Borrowed chords are another rule breaker, on the surface.  Borrowed chords refers to substituting one chord for another chord borrowed from another scale.  This is deeply rooted in jazz theory.  Here is a common progression.

I - vi - IV - V- I

Simple, and nothing out of the ordinary.  But let's change something.

I - vi - iv - V - I

We've change the IV chord to a iv chord.  How?

What we did was "borrow" the iv chord from the parallel minor.  The parallel keys are very closely related to each other, so naturally some chords can go back and forth without losing the key center.  The 6th of the IV chord changes to a b6th like in the parallel minor and viola! you have a borrowed chord.

You can do this with any chord you wish really.  There are a few standard kinds, which I talk about in the jazz theory portion of the site.