GIMP is a strong alternative.

GIMP is available on OS X, Windows, and Linux. GIMP is free and licensed under the GNU General Public License.

As for your feature list:

• layers

  Yes, gimp has this.

• all layer effects

  Yes

• patterns

  Yes

• gradients

  Yes

• smart objects

  Gimp does not have smart objects to my knowledge.

• history

  Yes

• PNG export

  Yes

• rescaling

  Yes

• text (and font properties)- paths

  Yes

• shapes

  Yes

• selections / color selecting

  Yes

• rotation

  Yes

_{Note: No two image manipulation tools are created equal. GIMP is an Image Manipulation tool at its core and, on certain levels, could be considered an alternative to Photoshop, but, as it stands, they are two entirely different beasts.}

GIMP is a strong alternative.

GIMP is available on OS X, Windows, and Linux. GIMP is free and licensed under the GNU General Public License.

As for your feature list:

• layers: ✓

• all layer effects: ✓

• patterns: ✓

• gradients: ✓

• smart objects: ☓ Gimp does not have smart objects to my knowledge.

• history: ✓

• PNG export: ✓

• rescaling: ✓

• text (and font properties)- paths: ✓

• shapes: ✓

• selections / color selecting: ✓

• rotation: ✓

_{Note: No two image manipulation tools are created equal. GIMP is an Image Manipulation tool at its core and, on certain levels, could be considered an alternative to Photoshop, but, as it stands, they are two entirely different beasts.}

GIMP is a strong alternative.

Gimp is available on OS X, Windows, and Linux. Gimp is free and licensed under the GNU General Public License.

As for your feature list:

• layers

  Yes, gimp has this.

• all layer effects

  Yes

• patterns

  Yes

• gradients

  Yes

• smart objects

  Gimp does not have smart objects to my knowledge.

• history

  Yes

• PNG export

  Yes

• rescaling

  Yes

• text (and font properties)- paths

  Yes

• shapes

  Yes

• selections / color selecting

  Yes

• rotation

  Yes

GIMP is a strong alternative.

GIMP is available on OS X, Windows, and Linux. GIMP is free and licensed under the GNU General Public License.

As for your feature list:

• layers

  Yes, gimp has this.

• all layer effects

  Yes

• patterns

  Yes

• gradients

  Yes

• smart objects

  Gimp does not have smart objects to my knowledge.

• history

  Yes

• PNG export

  Yes

• rescaling

  Yes

• text (and font properties)- paths

  Yes

• shapes

  Yes

• selections / color selecting

  Yes

• rotation

  Yes

_{Note: No two image manipulation tools are created equal. GIMP is an Image Manipulation tool at its core and, on certain levels, could be considered an alternative to Photoshop, but, as it stands, they are two entirely different beasts.}