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