What scientific plotting software is available?

I am a Physics student and found that the best scientific plotting software for Ubuntu is QtiPlot. It is very similar to Origin, and works really well.

Numpy and Matplotlib make a good combination for processing and displaying data.

I would suggest Gnuplot. It has a nifty set of features and is good documented. So if you take some minutes to skim through the documentation you'll get the basic idea. I use gnuplot for nearly all my plots, only when I don't need the full set of features I tend to use TikZ from LaTeX.