I use notepad on windows and on Linux text editor. It is fast and simple. It's really easy to use during meetings and it's extremely fast which is important for me. When task is finished I just delete the rows I don't need anymore.
I use one line for one task. If I need to write subtasks or some points I use tabs under that task. For different days I use visual separators like ---- or ===.
Example:
===Today===
First task
Second task
More info about task
Subtasks 1
Subtasks 2
===Tommorow===
First task
Second task