Microsoft has released a free 90-day trial for Windows 10 for those who aren't eligible for free Windows 10 upgrade and want to give it a test before buying. You can also install it in VirtualBox or VMWare for testing purpose. Only the Enterprise edition of Windows 10 is available for download as a trail which misses some new Windows 10 features like Cortana and Windows Store, including its modern apps like Mail, Calendar and Photos etc. Upon installation, you will be asked to activate your copy of Windows 10 but a product key is not required for the trial. After 90 days, you will start to see a desktop notification indicating that the system is not genuine, and the PC will shut down every hour.
Download Windows 10 Enterprise trial from here.