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.

Krita is a professional Free and open source (FOSS) painting program. It is made by artists that want to see affordable art tools for everyone withith concept art, texture and matte painters, illustrations and comics.

Pencil2D is an opensource animation/drawing software for Mac OS X, Windows, and Linux, based on old Pencil software. It lets you create traditional hand-drawn animation (cartoon) using both bitmap and vector graphics.

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. Draw you frame-to-frame animation from scratch, or create animations upon videos, images, PSD layers.

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