The name: GIMP
The site: http://gimp.org/

What it does: Allows you to make intricate graphics and save in many file formats, including Adobe Photoshop (.psd).
Is it free: Absolutely. Free download.
What operating systems does it run on currently? Check here to be up to date.

Here's a graphic I did to provide you with a simple example of what can be done with this program.