Photoshop (Image editing/pixel drawing application)

Illustrator (Vector graphics and illustration application)

GIMP (Free photo editing/pixel drawing application)

InkScape (Free vector graphics and illustration application)

Corel Painter (Pixel drawing/painting application)

Corel Draw (Vector drawing application)

Unity (Free game engine)

Sculptris (Free beginners 3D modelling application)

Blender (Free professional 3D modelling/animation application)

Printing On Demand Websites

Sell your own art and designs on T-Shirts, iPhone cases, canvasses etc.
The way that POD websites work is that you create and upload your own designs to these websites so they can be sold online on lots of different products. When a 
costumer buys one of your designs then the product gets produced ‘on demand’ and is then sent to the costumer by these websites. You will receive a percentage of the profit automatically so you don’t have to worry about anything but creating your art!