Inkscape is probably the most appropriate alternative for abode photoshop for vector related content creations. I used this open source app fro designing my app icons.
The app is a bit difficult to grasp, but you can use it after some learning. There are plenty of videos available in YouTube to guide you around.
These were the icons that I created for my app classify.
Classify - android, iOS
Do I think Modi is perfect ? NO
1 hour ago