What are the best Polarr alternatives?

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.

Adobe Photoshop is a raster graphics editor developed and published by Adobe Inc. for Windows and macOS. It was originally created in 1988 by Thomas and John Knoll.

Affinity Photo is a professional, full-featured raster graphics editor. Working in Affinity Photo is always live and you can pan and zoom at 60fps, with live previews and non destructive application.

Online photo editor, which can work with PSD, XCF and Sketch files (Photoshop, Gimp and Sketch App). Convert between PSD, XCF, JPG, PNG and many other formats.

darktable is a virtual lighttable and darkroom for photographers: it manages your digital negatives in a database and lets you view them through a zoomable lighttable. it also enables you to develop raw images and enhance them.

Create incredible photos anywhere with all-new Lightroom CC and 1 TB of cloud storage. Get powerful editing features on any device in a new, easy-to-use interface.

Pixlr® makes it easy to transform everyday images into stunning works of art. Whether you’re applying a quick fix to your photos or adding your personal touch with effects, overlays, or borders, Pixlr has everything you need to make your moments beautiful.

RawTherapee is a free RAW converter and digital photo processing software. All changes can be applied and batch processed at a convenient time.

PhotoScape is a fun and easy photo editing software that enables you to fix and enhance photos.

Glimpse is an open source image editor based on the GNU Image Manipulation Program. The goal is to experiment with new ideas and expand the use of free software.