GIMP stands for “GNU image manipulation program”, and it is one of the oldest and most well known alternatives to Photoshop in existence. Although it doesn’t quite have all of them, you’ll find most of the features included in Photoshop somewhere in GIMP. GIMP is cross platform and supported by a large community.
If just having the feature set isn’t enough for you, there is an alternative based on GIMP known as
GIMPShop. It’s the same as GIMP, except the layout has been structured as close to Photoshop as possible, so anyone making the transition should still feel right at home.