Good ol' pen and paper. Tried so many other things, but nothing works quite as well for me. I just want a to-do list that's easy to reference, easy to cross things off of, and no bells and whistles. Writing things down on a a notepad that sits next to me is simply faster than bringing up some to-do list somewhere else and referencing it. It's always there, right next to me, and all I have to do is glance at it to see what things I need to get done.