Some useful free software
Inkscape
Windows/Mac/Linux
An alternative to Adobe Illustrator. I prefer it!
OpenOffice.org
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.
Octave
Windows/Mac/Linux
A free clone of Matlab.
Firefox
Windows/Mac/Linux
The best web browser out there (in my opinion)
Thunderbird
Windows/Mac/Linux
A powerful email program with optional calendar abilities
Chrome
Windows/Mac/Linux
A web browser tightly integrated into Google
Paint.NET
Windows
An image editing program. May be used instead of photoshop for simple tasks, but it's nowhere near as powerful
Fiji (aka ImageJ)
Windows/Mac/Linux
An image analysis program.