Rectangle is a rewrite of Spectacle which is a popular MacOS app used for managing and controlling windows via keyboard shortcuts. While mostly the same as Spectacle, Rectangle includes some extra features like the ability to snap windows to screen edges. Both apps are free and open source.
Recently, the maintainer of Spectacle announced that they were no longer going to be able to support it, which is why I just switched. So, if you’re looking for a replacement, give Rectangle a shot. I’m liking it so far.
If you’re willing to pay, there’s no shortage of apps that fill this void. SizeUp is a good alternative, but it’s not quite as free and open source as Rectangle.