Since Adobe Illustrator, Photoshop and other software programs we use in class are so (relatively) expensive, here are a couple of sites you can access to download similar programs – or, in some cases, utilize on the web – that (attempt to) accomplish the same thing. Several of the descriptions were cut-and-pasted from the respective websites.
Inkscapeis a professional quality vector graphics software which runs on Windows, Mac OSX and Linux. It's free to download and is open source. whether you are an Illustrator, Designer, Web Designer ir just someone who needs to create vector graphics, Inkscape is as viable an option as any other.
Online Photoshop Freehas some of the same tools as an older version (less feature-rich) of Photoshop. You won't be able to do a lot of the "whiz-bang" procedures as Photoshop CS6, but for learning the basic tools this isn't a bad program -- and it's free. Online Photoshop Free uses the Pixlr engine to emulate Photoshop. Pixlr is a fairly feature-rich mobile app for Android (and iOS, I'm sure).
GIMP.org is another raster image manipulation software package that you can download for Mac or PC. It's "Open Source," so it will be updated frequently, and most likely remain free to download.