Ive been using window resizers for a long time.
In this domain (Window resizers), Moom was the original and absolutely great until Yosemite came along and hijacked the fill screen button (and turned it into full screen). Its now much more difficult to get the window resizing you want out of Moom. In the meantime, an application called Spectacle came along for free which allows you to resize your windows to your taste based on keyboard shortcuts (the fastest way to deal with window resizing). Spectacle allows you to install it on any computer with no login, no registration and no payment. Works from 10.6.8 on (different versions). Code is open source on GitHub. Magnet is just an opportunity to pay for what is amazing and free (Spectacle).
Uncoy about Magnet, v1.10