Sketch should be pre-installed on your Maemo device and can be found in the applications manager under... well, your local name for "Sketch". So maybe... ébauche or something like that?

Though, which application would be the best depends on what you want to use it for.

Sketch is perfect for taking short (short!) notes, for example. If you want "true" handwriting (something that also looks like your actual handwriting) on an expandable canvas, MyPaint might be a better choice, but it might just as well be a bit overkill. I don't know about PenPen, but provided from the description it sounds rather interesting.

I guess you'll have to download all of them and just give them a try to decide which one really suits your needs.
(And I'm going to download PenPen now - that sounds REALLY interesting! )