Windows 10 and Windows Insider Program

Windows 10 Technical Preview is available for free to try and give your feedback to Microsoft.

You need to sign up for the Windows Insider Program and download Windows 10!
https://insider.windows.com

This preview has features like the new Start menu, the multi-desktop functionality and more.
A different preview made for consumers that will add features like the "Continuum" touchscreen mode will be released in early 2015.

I suggest you watch this small intro video:

Introducing Windows 10 - the best Windows yet

WARNING: Trying out an early build like this can be risky. It is *NOT* recommended to install the preview on your primary home or business PC. Unexpected PC crashes could damage or even delete your files, so you should back up everything.