TL;DR: AltDrag is being actively developed again on a new fork and now works flawlessly on Windows 10!
AltDrag is one of those "so obvious in hindsight" tools that implements functionality that really should be built into Windows – the ability to move and resize a window by holding [ALT] and dragging it with the mouse. It makes managing lots of windows so much easier (and faster) and can really speed up a lot of workflows. Lots of Linux desktop environments support it natively, and I've always been surprised that Microsoft never added it to Windows.
Thankfully, AltDrag came along as a free, open-source tool to bring this functionality to Windows. It was originally developed for Windows XP and updated for Windows 7, but eventually the developer's real life obligations started to encroach on his free time and the tool ended up on "life support" as he no longer had time to keep it updated.
When Windows 8 came along with Microsoft's first genuine attempts to handle high-DPI displays (i.e. 4k monitors), AltDrag ended up having trouble with non-100% scaled windows (which ends up being pretty much every window on a 4k monitor) because of how it works. A workaround was eventually found, and even after Windows 10 broke it again, another workaround was found that kinda-sorta fixed it. The tool itself didn't get updated; the workarounds both involved using various settings in its Explorer properties "Compatibility" tab.
In Windows 10, even with the workaround, it was still flaky, and for a good long while, it seemed like it was never going to be fixed. Thankfully a mad lad going by the name RamonUnch on Gitlab decided to fork the tool and update it for Windows 10, resurrecting it and bringing it back from life support. You can get the new up-to-date version of AltDrag from GitHub.
Go grab it and improve your Windows experience with one key and a click!
NOTE: Windows 10 might flag altdrag.exe as malware after installation (it calls it a "severe" threat and automatically quarantines it). It gets pissy because AltDrag does fun things with keyboard & mouse inputs and messes with, well, potentially every program running on your system based on your actions. For once, I actually don't object to Windows presuming that it's malware, because generally speaking you don't want programs to be doing this sort of thing. But since you're installing AltDrag deliberately and know what it's doing, you can safely use the security panel to restore it and grant it an exception.