I highly recommend Swish as an alternative to Rectangle. Recently discovered it and wouldn't go back. The main thing that sold me is the ability to resize two windows by dragging the divider between them (similar to how Windows works).
This looks neat, but it only seems to be useful for touchpads, which is the vast minority of users. And while it's not expensive, it's also not free. Any software I have to manage with an extra license is an added time burden. (even if minimal, it adds up)
I've been using Spectacle for a while now, https://github.com/eczarny/spectacle, it seems to be less feature rich than some of the alternatives other people posted but it's free and works well for what I need it to do. Basically, just keyboard shortcut to move windows do different monitors and resize them to the left, right, or fullscreen.
Ah, didn't realize that. I've been using Spectacle a lot longer than the 3 years since they added that. I guess I'll be sad if it ever stops working or I'll switch to Rectangle.
I've been hesitant about installing software like the previous suggestions, so I opened the Stickies app and resized them to create a guide. I like to have a space between windows so that the wallpaper is still visible in some of my Spaces.
My issue is that it's not efficient and I need to nudge some windows from accidentally moving them.
I don't understand why Apple didn't build this functionality and enabled it by default. It's much easier dragging a window at any corner or side on a Linux/Windows OS.
[1] https://rectangleapp.com/
(I add this to all the Macs I touch)