Sage is an open source, Python based computer algebra system that includes features from many different open source mathematics packages.
Sage is installed on the leibniz server and can be accessed from any web browser at https://leibniz.math.isu.edu:8000. See the page about leibniz for information on requesting an account.
Once logged into Sage, you will be presented with the Sage notebook. Workbooks can be added to the notebook, with each workbook consisting of a series of Sage commands. Workbooks are similar to Mathematica or Maple files.
To learn more about Sage, please take a look at official Sage Tutorial. Sage also comes with documentation built directly into the Sage source code, which can be viewed by placing a question mark after any function or object (e.g. plot?). See the documentation on getting help for more information.
Because Sage is essentially a Python interpreter heavily augmented for math, resources on the Python programming language should also be of some use, especially when debugging errors.