From what I’ve seen, most folks, while writing lists in sentences, write them as “x, y and z”. I, however, write them as “x, y, and z”. The difference is that last comma before the “and” in those lists. (Yes, I know that I committed my favorite error in those examples above.)
That last comma is usually called an Oxford comma or serial comma. Wikipedia says it can also be called a Harvard comma, but, as an MIT alum, I think Harvard is quite obnoxious enough already without a comma named after the place.
Overall, I’m a big fan of writing (a) as I speak, and (b) as clearly as possible. When I am verbally using a list, I pause for the same amount of time between saying “x”, “y”, and “and z”. So my brain says that I should use the Oxford comma when I write the list. I also find that, in most cases, using the Oxford comma makes a list easier to read–it reads clearly as a list to my eyes.
The Wikipedia entry has all sorts of fun ambiguity examples that I’ll refrain from duplicating here. I have to say that, despite my love of this comma, I no longer correct it when I proofread documents from authors who don’t use it. I barely twitch, even. Does this mean I’m gaining maturity?