"impressed with" and "impressed by"

What is the difference between "impressed with" and "impressed by" ?

Perhaps the easiest way to answer this question is by giving an example sentence. Look at the following:

Albert impressed me with his ability to analyze a problem.

We can also say:

I was impressed by Albert. (Who impressed me?)
I was impressed with Albert's ability to analyze a problem. (How did he impress me?)

It should be added that while this illustrates the differences between the two ('by' for the person or thing doing the impressing and 'with' for how), in many situations there is an overlap in meaning and either may be used.