Plotting

Python:

Useful tutorials/references for python and plotting in python using matplotlib.pyplot:

SciPy plotting tutorial (Very basic, includes all the commands for the simple case.)

Pyplot tutorial (Still fairly basic, includes more plotting examples with excerpts from the code and links to the full plot example source codes.)

Pyplot screenshots (A gallery of plotting screenshots and links to example source code.)

Pyplot gallery (A thumbnail gallery of links to different plotting examples.)

Matplotlib plotting commands (A full listing of matplotlib plotting commands, with links to the package docs.)

Wolfram Alpha:

Free, somewhat less effective version of Mathematica online.

Sage Online notebook:

(Free account required): Sage notebook.

Useful things:
Basic plotting example
More plotting + other stuff (like the kitchen sink)