I use Evernote. I wrote a hub about keeping my class notes organized using this program:
I use Evernote to keep track of class notes, recipes, to do lists, blog ideas, etc. I use it every single day.
I also use Google Calendar which I sync with Calendar on my Macbook and iPhone. I have reminders set so that my rent is never late and I never miss a due date or appointment.
Lastly, I still love to hand write to-do lists. I can't help it. I think it's probably my obsessive compulsive personality but I write out numerous to-do lists every day (writing and re-writing them). You can tell how stressed out I am by how many to-do lists I pull out of my pockets at the end of the day.