Apps with 'Symmetry brush' feature

GIMP is a free program for such photo retouching, image composition and image authoring. Essentially it's the closest thing to Photoshop you can get for free.

Professional, open-source raster graphics editor with advanced digital painting and sketching tools and workflow features.

Animation Desk is a perfect tool for animating, storyboarding, and sketching. It’s an app Loved by professional and amateur animators, Youtubers, and many art lovers.

Pencil2D is an opensource animation/drawing software for Mac OS X, Windows, and Linux, based on old Pencil software.

Generate tags for Youtube Videos. Works as a browser extension.