Unfortunately, all of the core applications on the phone are closed source. The settings, media player, image viewing, camera, phone and email applications - all proprietary software that was not open sourced by Nokia.
Now, 4 years after the purchase, I'm stuck with a broken note taking application: When loading a notes file with bullet lists, I get an empty item added after each "real" list item:
This bug is known since 2009 and has 4 duplicates, but since Nokia stopped the development on Maemo, the issue will never be fixed.
And again I am severely limited by the unavailability of the source code (look for osso-notes), forcing me to either abandon the phone or to find a replacement application.
Alternative to Maemo notes
After being annoyed about that bug for about two years, I searched the bug tracker, found that there is no fix and looked for an alternative. I found Conboy, a nifty little note taking application that can even be synced with the desktop.
Its file format is Tomboy's XML, which is - while not normal HTML as osso-notes wrote - still open enough to be useful when something really goes wrong.