A few weeks ago I had my (2 years inactive) WoW account get owned and banned, possibly through my email account, so that was a major sign to sort out and properly tier all my passwords. I found firefox's list of saved passwords to be particularly helpful as a checklist of sites to change, as well as a reminder of how stupid I had been using my "good" password on far too many low priority sites in the past. Also a strong reason against having one "good" password.
Thanks to your post, however, I am also reminded that I shouldn't assume this list is complete, as I had completely forgotten about Ebay and Paypal passwords, which I must not have used in the past couple years.