[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**: ✓

 - **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**: ✓

<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>