Windows: reSizer is a tiny utility that moves, resizes, repositions and tweaks the appearance of open applications and windows with keyboard commands and hotkeys. As long as the app is running, you don't need to take your hands off the keyboard just to reposition one window next to another or change the transparency of the window on top.
reSizer supports multiple monitors and runs in the background once installed. It comes with a number of hotkey presets to make it easy to get started. For example, to bump a window to the lower-left corner of your primary display, press Win+Numpad 1, and to move a window in any particular direction, press Win+Arrow Key, with the arrow key being the direction you'd like to move the window. To resize windows, you can press Win+Ctrl+Arrow Keys with the arrow keys corresponding to the side of the window you'd like to expand. Similarly, Reszier can also change the opacity of the active window by pressing Win+ / (slash on the numpad.)
Granted, reSizer doesn't do anything you can't make AutoHotKey do with scripts and tweaking, but if you don't want to spend the time setting it up, reSizer is a quick install with minimal setup. Once it's installed, you can get started immediately and get back to work. Do you have a preferred way to resize or move windows with the keyboard? Share your tips in the comments.