let me add a couple more
* one you already know of is Notepad
from Cosmos
Just download the Alpha or stable zip package (not the exe) and take only the Notepad folder.
It is perfectly working standalone, and is a nice no-frills ingame Notepad with few if any bugs.
(Notably only the truncation of long files which is a WoW editbox bug more than an addon bug)
* second is NotesUNeed (or NUN for short) which can be found
on this here site also .
This is a very feature rich Notepad application with many automated note taking features and integration with MapNotes etc.
(if you have any problems with the latest version visit curse and take the last stable 1.x version, this is also maintained as a lighter version and is extremely stable)
Hope it helps.