Sandra complained about compatibility issues while teaching an online course, and in response, I and several others recommended using Open Office instead of excel. She responded with a list of reasons why she prefers excel, which included the use of pivot tables to count unique items in a column.
To prove that this is easy to do without using pivot tables in OO Calc, I whipped up a short screencast_
It's a little small, but you can see the details by viewing it full screen.
I used recordMyDesktop to do the screencast, which makes it dead simple.