For anyone looking for a free alternative to the ones already mentioned, Taekwindow is worth checking out.
Besides the brilliant name, it offers some extra functionality that you might deem important:
- move a window by grabbing it anywhere (not just the title bar) while
holding the Alt key, and then dragging with the left mouse button;
- resize a window by grabbing it anywhere (not just the tiny little
border) while holding the Alt key, and then dragging with the right
- move a maximized window between monitors by
- use the scroll wheel on the window under the cursor,
instead of the currently focused window;
- push a window to the
background by middle-clicking on its title bar.
You can quickly enable/disable the program by left-clicking on its icon in the systray. Personally, I found it one of the most lightweight apps that offers the requested scroll-under-mousepointer functionality, as well as one of the most stable. It's a minuscule download (28K) and doesn't require installation.
I'm not affiliated.