Google Apps Is a great suite of tools for secure mail, website, Create, Word, PowerPoint and Excel document. Great for mobile devices.
Tux Typing Is a fun way to improve your students keyboarding skills. Great for pre-schoolers to adults.
Tux Math A fun way to reinforce mathematics for pre-schoolers through Jr. High.
Tux Paint One of my favorites. A great way to inspire your student and develop their artistic abilities and learn their way around a computer.
GCompris Suite I even dabble in the physics games on this fun early development tool.
Gimp A free imaging tool that is just a powerful as expensive adobe products.