ReactOS is an open-source binary-compatible reimplementation of Windows.
In other words, it's free. You can boot your desktop or laptop from it. It looks like Windows (a 10-year-old version, anyway), so you already know how to use it. And it'll run some Windows and DOS applications, maybe including DOS games that regular 64-bit Windows can no longer touch.
While that sounds great, this isn't a project for the average user. It's an alpha, missing some major features (they've only just added NTFS read support, can't write yet), and what you do get has plenty of bugs.
If you're willing to spend some time exploring ReactOS, and you don't expect too much, then there are some interesting touches here. Double-click the Applications Manager shortcut on your desktop and you're able to browse and install some great tools, all in a couple of clicks: VLC Media Player, Winamp, GIMP, Irfanview, Firefox, Opera, OpenOffice, BurnAware, CCleaner, assorted torrent clients, VNC tools and more.
You can also try installing other Windows and DOS applications. This is more likely to work with older, simpler packages (Win95 text editor = probably yes, huge multimedia suite = probably no), although there are no guarantees.
For the best results, follow the download link, grab the "BootCD" file and install it somewhere (this can be a VirtualBox VM if you don't have a spare PC). There's also a LiveCD which will boot ReactOS, and allow you to browse its menus, but you won't be able to install anything.