For those that (like me) always have a Terminal open anyway, and that are looking for an easy way to make notes, try this alias:
alias note 'date >> ~/unix/notes.txt; tee -a ~/unix/notes.txt > /dev/null'
This alias appends date and time information to the file described, and then appends everything you type in the terminal until you stop with Control-C. The
tee command actually forks input to a file and to standard output; we don't need the latter, so we send it into oblivion. Obviously, you can have several of those writing to different files. To me, this is easier than opening a text editor.