Been using a spreadsheet here for the family finances for about 7 years. Works great!
I keep a note in my iPhone. trusted to have like 17 bills a month...now that I've paid down debt and limited my credit card usage i have 3 bills. Cell phone, cable for my girls family and the one credit card
My boyfriend and I use the personal finance, money management software Page Once. Theres a web app as well as mobile apps for every mobile platform - I can only vouch for the Android and web apps.
We both added all account's to our Page Once app and we keep track of due dates, account balances for banking, investments etc.. and we know when and whether an account was paid without asking the other.
The other plus side is I can also manage my student loans and keep track of all upcoming flights and all the air fare milage I build from them since my career requires me to travel a lot.
I still do it the old fashioned way, ( I'm 71 ), I keep everything in my head. I only have about 7 bills to pay monthly, including utilities. I only have my computer, no cell phones, ipods, ipads, etc. When I get a bill, I go on line and pay it right away. I don't know if my brain could keep up with some of the bills you younger people have. I give you all credit for being able to keep up with it. A lot of my spending is still on a cash basis.
I have most of my monthly bills either charged to a credit card (Direct TV, Insurance, Verizon) or automatically paid from my checking account (gas, electric, water, car payments, mortgage) so I don't have to worry about those. I have all of my credit cards set up to email me payment due reminders. When I get the reminders, usually about 1 week prior to due date, I go in and set up the payment to be drawn from my checking account on the due date. I also have some of the ones I don't use often to email me the balance due on a weekly basis.
When I am working to get my FICO score up as high as possible, I get a little more proactive and monitor the credit cards weekly and make payments that maximize my FICO (paying most off before statement close date etc). I have about 20+ monthly bills including my son's college expenses (apt rent, utilities etc) and I just make sure that I take at least 1-2 days a week to monitor the accounts so I don't miss anything. I have my checking account ledger in Excel rather than using a check register so it is easy to check what was paid when in prior months.
Guess maybe i am kinda old fashioned. I just a simple wall calendar next to my computer desk. Since at some point I will use my computer everyday I will always look at it. I also have the same dates in my phone for backup incase I am stuck at work for two weeks or more.
I have a pretty good idea of how much each is costing me and I balance it out so that the payments are paid out on my pay days evenly. Even my gas for the vehicle is regular - fill the tank every 10-12 days! There is no need to pay exactly on the due date as long as you each bill regularily and within the payment period. I know how much "mad money" I have left over for other expenditures. Hint: I am also not married so I am more predictable in my expenditures!
That's 14 autopayments per month and a couple other manual payments. I actually do very little "tracking" of my payments as they take care of themselves once they are set up. I just need to do a little gardening now and then to keep everything in line. Note: I have also been doing all my banking on line since 1994. I can't remember the last time I actually saw a CSR in person!