Apps with 'Curve Manipulation' feature
Inkscape is a professional vector graphics editor for Linux, Windows and macOS. It's free and open source.
Paint.net is a free and very capable image editing software for Windows. Great alternative to Photoshop for people that do not need all the stuff PS offers.
RawTherapee is a free RAW converter and digital photo processing software. All changes can be applied and batch processed at a convenient time.
PaintTool SAI is high quality and lightweight painting software, fully digitizer support, amazing anti-aliased paintings, provide easy and stable operation, this software make...