Based on your search for a free, open-source spreadsheet software, I recommend GeoGebra. While it's primarily a dynamic mathematics software suite, it does include a built-in spreadsheet feature for data management and calculations. This could be a great tool for you if you're teaching subjects that involve data analysis or mathematical computations.

Here are some key features of GeoGebra that might be useful for you:

• Interactive Geometry Environment: This allows you to demonstrate geometric transformations such as translations, reflections, and rotations.
• Computer Algebra System (CAS): This built-in feature allows for symbolic computations, which can be useful in teaching algebra and calculus.
• Statistics and Calculus Tools: These built-in tools can be helpful for teaching statistics and calculus.
• Scripting Hooks: For advanced users, GeoGebra provides scripting hooks that allow for customization and automation of tasks.
• GeoGebra Materials Platform: This cloud service allows you to upload and share GeoGebra applets, including interactive worksheets, simulations, games, and e-books created using GeoGebraBook.

GeoGebra is free for non-commercial use by students and teachers, making it a cost-effective choice for your needs.