BillMinder is a simple app for iOS devices that allows you to enter any number of recurring bills and remind you before they are do. Hence the name "BillMinder," obviously. I use BillMinder to alert me and will say that despite a few limitations it does a great job of reminding me of upcoming bills. You can see a full demo in the video below, but it's a fairly straightforward app.
In BillMinder you set a recurring date, the amount of each bill and the interval you'd like to be reminded of said bills. I have mine set to one week before they are due. Yes, you could do this with a calendar app, but then you have bills littering your calendar and no real accounting of how much you've paid.