As you move windows, Gridy snaps them into place along invisible lines, much like that used by the "Align to Grid" option for arranging your desktop icons. The small, simple utility runs from the system tray and includes a few other features, such as making a specific window transparent or removing it from the alt-tab list of open programs (which also changes its border to remove the minimize and maximize buttons). It's likely best suited for someone who frequently lines up windows for side-by-side comparisons and other such work.
Double-clicking the systray icon displays all the hotkeys for additional features, and right-clicking the icon allows you to change options such as the size of the grid. If you do change the grid size, keep in mind that the displayed numbers are for the cell size, so choosing a larger number means fewer and more widely spaced hidden grid lines.
Gridy installs quickly and asks you if you want it to start automatically with Windows during the installation. A is also available.