Some useful free software

Inkscape

Windows/Mac/Linux
An alternative to Adobe Illustrator. I prefer it!

Windows/Mac/Linux
As the name suggests, it's an alternative to Microsoft Office. It's not quite as powerful, but for most people it should be fine.

Windows/Mac/Linux
A free clone of Matlab.

Windows/Mac/Linux
The best web browser out there (in my opinion)

Windows/Mac/Linux
A powerful email program with optional calendar abilities

Windows/Mac/Linux
A web browser tightly integrated into Google

Windows
An image editing program. May be used instead of photoshop for simple tasks, but it's nowhere near as powerful

Windows/Mac/Linux
An image analysis program.