Notices

Spacetime is the only piece of software which still makes me keep my old XDA Flame. It's a powerful graphing calculator with numerous features. I've tried SlideRule, but it's way behind Spacetime. Is there anything I missed?

Try Almost TI85.

I also use Space Time, but I don't think that you'll find substitute on Maemo... SpaceTime is commercial app, that'v been worked on really hard... But if you find something, let me know please.

Almost TI85 is very good, however it's still not as good as Spacetime.

Spacetime for Windows Mobile has recently been made freeware, probably due to WM dieing and WP7 coming in a few months. But maybe if we gather more users, they will make a port for Maemo, or at least Meego? There is also a second way we could run Spacetime on N900: as Maemo isn't very popular (even less market share than WebOS) the chance that there will be a version for Maemo is small, but since Android is gathering more and more users, they will eventually make the version for it. Heck, it's available for iOS, but Android is catching up and in some time Android are going to outnumber iOS.

Well if they port it on Android, I don't thing that Android on N900 will do the trick... I didn't tried it yet, but it seems really slow and kinda painfull for N900 HW. Drivers on Android for N900 are poor and from what I'v seen and read, the community around N900 is dying...

What I'm trying to say is that I don't think ST will be usable on N900. I personally bought iPod Touch mainly coz ST, so my iPod is basically big, fancy calculator only

If you wanted only graphing calculator, couldn't you just buy Classpad 330, HP 50g or TI-Nspire CAS? They are the best graphing calculators and have lifetime of about 10yrs, which can't be said about iPod Touch.

Why do you think the community around N900 is dying?

Note freeware not opensource. One of various reasons for changing a program from "commercial" to "freeware" is end of life - ie probably little commercial value left.

My HP28S is still going strong after 21 years

Apple will probably have stopped making batteries for them even if your device is still working.

there is always gnuplot for the n900. desktop quality graphs.

http://www.gnuplot.info/

Maybe a mix of GNU Octave (for computation) and Gnuplot (for plotting) would do the trick. You'll probably have to install Easy Debian and grab the Octave package from there, unless someone ports Octave to Maemo.
Be warned, though : these are command-line only software, so you won't have the Spacetime look and feel. However, those tools are really powerful once you mastered them (and they are available on a wide range of hardware and OSes, so they are worth the time it takes to learn them).

I've installed gnuplot, but didn't mention it here, as it only plots, but it's really good. As for GNU Octave, I'll install it later when I have time.