I have used a "ToDo List" for a very long time. I suppose that it initially started by using a list written down on scraps of paper, and progressed to writing things down in a very small pocket-sized notebook when I started working on the Wards. This was very useful as an 'aide memoir' for a busy shift, and even was useful in doing Consultants ward rounds, or helping to manage the ward when it was my turn to be Team Leader, or in charge of the ward.
Whilst at home I still used paper todo lists usually written on the backs of envelopes but it was only when I started using a computer that things became interesting. When I used my portable Atari computer, I used "Brown Bag Outline" and stored all the information on floppies. This was before the days of the 5.25" floppy disk or CD's or even DVD's. I can't remember what I used when I was using "Windows 95" or "Windows 2000", but when I started using linux I used "emacs".
And when I started to use a mobile phone that you could install small programs on, and I phrase it like this as my first mobile phone was the size of a small brick and could be carried in your handbag or clip it on to your belt. It had a very short battery life and the range was limited, and the coverage of the radio masts was very poor. You generally had to go outside to make a call on it, as the weak signal meant it was difficult to use indoors.
Anyway, I'm currently using Todo.txt on my mobile, which is stored in my dropbox, from where its accessed by emacs and displays it in a separate buffer, where I can read it and cross the tasks off when they're completed. This works extremely well for me, but I've been thinking about how other people use 'ToDo Lists' and looking for more information about the whole subject. Hence these series of articles.
A very good site for starting off with 'Todo.txt' is Todo.txt where you can find download links for an IOS app or the android app. Its also got hyperlinks for other useful sites, and to the github site for the source files.