I have gmail and thus use a lot of Google's other services. I love the reader to keep track of all the blogs I read, etc. A few months ago, I decided to give the calendar a whirl. I now use that to keep track of my bills. When I get one in the mail, I go to my calendar and create an event for the day it's due. I've got it set to email me a reminder a few days before it's due so that I can get online and pay it. I did set my calendar to be private and protected, but even if it wasn't, I don't include things like account number. I just abbreviate who it is and the amount.
Now if I just would remember to ALWAYS log my bills I'd be in good shape. ;)