UPDATE 2024: Get a new version here: https://github.com/ivoras/EncryptedNotepad2
"Encrypted Notepad" does only one thing, but aims to do it perfectly - a Notepad-like simple text editor where files are saved (and later loaded) encrypted with industrial strength algorithms. Available both for the desktop (as a Java application) and on Android.
Unencrypted data is never stored on-disk. Source is released (on SourceForge) under the BSD license.