Product review (5 stars): FIXED!
I had a lot of trouble tracking down the right part number to order, because Whirlpool changed their numbering system at some point after my washer was built, but when I finally figured it out, I sure was happy to find out I could get it from Amazon.  Anyway, this made the agitator in my washer work like brand new (it wouldn't turn if the washer was over half full of clothes.)  The included instructions were useless, but if you can take something apart by looking at it you'll be fine.  There's also a great instructional video I found online, from a popular video website.