Windows 10 Technical Preview
Today Microsoft launched their Windows Insider Program that allows you to sign up and download the Windows 10 Technical Preview. The program also allows you to take discuss Windows 10 with Microsoft staff and your peers in the Technical Preview forum. Once you join the program you will be able to download an executable that can upgrade your existing Windows installation. If you wish to install the Windows 10 Technical Preview in a virtual machine or in a clean setup, you can download the Windows 10 TP ISO instead. The technical preview is based off of Windows 10 build 9841 and will expire on April 15, 2015.
As this is a very early release of Windows, it is not suggested that you upgrade your normal computer to the Windows 10 TP. Instead you should use a spare computer or even better a virtual machine like VirtualBox. If you decide to go the VirtualBox route, download the ISO and create a Windows 8.1 guest in VirtualBox. Then go into the settings of the guest and mount the ISO as a DVD. When you are ready to install, double-click on the guest to start the installation from the mounted ISO. It will install perfectly, but unfortunately at this time the Guest Additions will not work. I expect a VirtualBox update will be released soon that will allow the guest additions to work.