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Well, I use a combination of four things:

First, I tend to store my long term ToDos in iCal, these are things like my resolutions for the year, or DVDs and books I might want to buy. I also put important date specific todos in iCal.

Then I use the iCal Event widget to make sure those are quickly accessible, I also have iCal load up on startup.

In addition, I use the dashboard stickies for mental reminders on general todos that I need to do daily, things like exercise, study for a certain class, exam dates.

Then I also have a paper list, for groceries I need and the stuff that I need to do in the next day or two.

Hope that is at least somewhat helpful.
 
I've just started a job in which I use a PC all day, and apart from the fact it makes me want to claw my eyes out repeatedly throughout the day, I must say that Outlook is rather good for this sort of thing. It is inferior to Mail.app for managing email, and it isn't as good as iCal, and so on, but the intergration between the equivalent of Mail, iCal, Stickies and a tickable tasks "to do" list, all viewable on a title screen is helpful.

As for what's available on a Mac, I did come across Inbox, and posted a thread about it here, and thought it looked interesting, but I haven't looked at it for ages.
 
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