Need a free replacement for Windows 2000 or later? Then maybe ReactOS could help --- in a few years from now.
It has been a long time coming, and the wait continues, but if you're feeling lucky, then ReactOS 0.3.14 could be a solution for your old Windows installs of the past.
Beginning in 1996, but still clearly in alpha phase, ReactOS aims for binary compatibility with Windows NT 5.0 (Windows 2000) and later releases. It does this by working with the WINE project, but it believes that WINE will never be a full replacement for Windows, especially when it comes to Windows drivers.
There are a number of restrictions, such as only being available in 32 bit, plus a lack of NTFS support, as well as the general alpha state of the distribution.
It's one OS to keep an eye on for the future -- it would be nice to one day replace an ageing instance of Windows XP with ReactOS.
If you are feeling game, then give it a try.