Well, it's not OpenSource, but my all-time favorite program, EccoPro, was discontinued in 1997 but made freely available. It runs on WINE on a desktop and is not too "heavy" a program -- no surprise since it's so old. Here is a thread from this month about running EccoPro on Ubuntu.

I and many others still rely on Ecco Pro, and not because our data is stuck in it, but because no other personal information manager compares. While it used to be hosted free, these days it can be downloaded by buying a $10 membership to the website/forum that hosts the software. Or if you PM me I'll email you a copy.

It's the most award-winning information manager in history. If you have any interest, start with this article.

I know that it ran on WINE long ago. Since it's rather lightweight and cleanly coded, maybe it will run on WINE on a Tablet/N900. I'd sure be interested to find out!

[That is, I'd be interested to find out, but I've never installed Linux anywhere and own an N800, not an N900. My interest in WINE arises from my interest in EccoPro; my notion is that if I ever leave Windows XP, I'll go to Linux, but I'm taking EccoPro with me!]