Some years ago I came up with a system which I still use extensively. I added these lines to my .bashrc file:

  log(){
  pushd ~/logs;
  echo >> $*;
  date >> $*;
  cat >> $*;
  } 

So whenever I have a piece of information that I'd like to keep - a thought on a project, a word from a foreign language I am learning, ideas for christmas gifts, some bash-fu and so on... - I fire up a terminal (mod+return, in my case) and enter

  log filename

Now I can type down that piece of information right away. File names could be "english" (for vocab) or "xmas" (for gifts) or the name of a project. So either the file already exists and I thus add to it or a new file is created instantly.

It is super quick and non-distracting (i.e. I don't 'accidentally' get carried away by looking at previous thoughts or tweaking the config of a journal application etc.) What's more I find it very convenient that the function automatically adds the current date to each entry.

Btw, I've got another function called "show" which I can call whenever I feel like just briefly skimming through a file

  show(){
  cat ~/logs/$*;
  }

For my daily todo list I enjoy abusing markdown syntax. I do this because syntax highlighting makes it easier to quickly scan through the files. (I use gedit with the monokai theme)

So at the beginning the list looks something likes this:

Task 1

Task 2

<!-- some details on task 2 -->

Task3

Task4

<!-- some details on task 4 -->

(comments turn gray so that actual tasks stand out)

Whenever I begin a task I place a "-" in front of its line. Now I see this red minus and thus know that I have started a task that needs to be completed. When I've finished the task I replace the - with a # and now the whole line turns blue. I've line numbers enabled in gedit so when I want to change the status of an item I simply hit ctrl+i and enter the corresponding line number. Some tasks occur daily, at least for a while. In that case I revisit a task's line again at the end of the day and remove the "#". Similarly, when I want to delete an unnecessary task I go to its line and hit ctrl+d and thus delete it. It is super primitive but I like it. ;)