The best alternatives to Magnet are Spectacle, LayAuto for Mac, and BetterSnapTool. If these 3 options don't work for you, we've listed a few more alternatives below.
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Spectacle is a simple utility that allows you to easily organize your windows without using a mouse. Those familiar with similar apps will find Spectacle to be a simple and unobtrusive way to become more productive while using your Mac.
BetterSnapTool allows you to easily manage your window positions and sizes by either dragging them to one of the screen’s corners or to the top, left or right side of the screen. This enables you to easily maximize your windows, position them side by side or even resize them to quarters of the screen.
⦿ Snap windows by pressing a modifier key and moving your cursor ⦿ Snap windows with keyboard shortcuts or by dragging windows to the edge of the screen ⦿ Move & resize windows by holding a modifier key and moving your cursor ⦿ Snap windows not in focus