Wunderlist is a task management tool that syncs across multiple platforms and even has a smartphone version! I use both the Mac OSx app as well as the iphone app. I enter tasks in a variety of lists, entering in rough dates i hope to have each task done by. Every day I pull up the app and open it to the "today" tab to see what's on my agenda. I can feel pretty good about myself when everything on the "Today" list is checked off, and I can move to tasks I didn't set dates on.
Wunderlist is also cool because you can share lists with other people, and it syncs straight to my phone. Note I have a grocery list on the right. I can add stuff to it as i think of it, and then when i'm at the store i can load it up and not forget a thing. Pretty sweet!