ReactOS is a recent open-source clone of Windows NT that has for many years been in beta, and as "0.4" must tell you, it is nowhere near complete yet.
But of course, as you must imagine, this is a massive undertaking. And considering how much Microsoft loves to add and change things in Windows, it will always be exciting to see what comes next in ReactOS.

ReactOS 0.4 is the first real major milestone, other than the initial release ten years prior, of ReactOS, and includes several new features. Some of these features include:

• Reading and writing of ext2 filesystems
• Reading of NTFS filesystems
• Brand new Explorer shell
• SATA, USB and networking improvements

A full list of the changes can be found here.

Here is the splash screen. Nothing has really changed here since 0.3.14.

Oops.

Here is the default desktop, albeit with the wallpaper turned off. No real change here either, although this is a good thing, as compared with Microsoft who thinks it is fun to change the entire appearance of Windows with every new release.

The start menu still behaves like it should.