Microsoft Word, as an example, even has an option to toggle whether commas are suggested before the last list item in its grammar settings.

To be honest, you can nearly throw it in wherever you want to cause a pause, but most people use far too many of them. Myself included.

In the first one there is a pause that adds emphasis to “and co-workers”, in the second there is not, and it lumps them all with equal importance. If that makes any sense.

I would have written the sentence a little differently, to avoid the comma confusion that really exists in both forms.

Friends should come after the other two, in my personal opinion. Employers and Co-workers are related, friends are intra personal.

I would write it as:

Employers, co-workers, and friends consider me a motivated person. (Direct, and flows more easily than the previous version)

Or keeping current form:

I am considered by *my* employers, co-workers, and friends to be a motivated person. (So version 1, I guess.)

But I really prefer wording it differently entirely.

In general, if it’s just a list of reasonably simple terms, there should not be a comma before ‘and’.

However if it’s a more complex list, perhaps where the terms contain multiple words, it can be acceptable to use a comma before ‘and’. This is sometimes known as an ‘Oxford comma’.

For very complex lists, where even an Oxford comma doesn’t help to remove any ambiguity, there is the option of using semicolons instead of commas to separate the terms.

Read it aloud to try to judge where the comma comes in, though that’s sometimes difficult with a list!

http://en.wikipedia.org/wiki/Oxford_Comma