[GIMP](http://www.gimp.org/) is a strong alternative.

GIMP is available on OS X, Windows, and Linux. GIMP is free and licensed under the [GNU General Public License](http://www.gimp.org/about/COPYING).

![GIMP][1]


  [1]: https://i.sstatic.net/ycAkg.png

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

<sub>**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. </sub>