Personally I use the Notes app available on the MacOS / iOS platforms. I organize it with a folder per project. I love it because it is simple, supports checkboxes, formatting, freeform drawings via iPad+Pencil, syncs across all my devices, allows password locking certain notes (though I don't really use this - I use 1Password to hold sensitive info via its Secure Notes feature) and is even available via the iCloud website if needed.

Within that folder I keep one note that acts as the ToDo list / Work Log that contains all the work items plus dates of completion. The rest of the notes are ad hoc and usually contain stuff like "Meeting notes - <subject> - <date>", "Follow ups", "Questions/Clarifications for later", and so on.