Windows Insiders are getting a number of new features to test, including a clipboard history, making it much easier to copy again, and again, using timeline sets, meaning you can copy across any PC (running this build or later).
File explorer also now has a dark theme mode.
Extended line endings support for Notepad include CRLF for Windows, Unix/Linus line endings (LF) and Mac line endings (CR).
There's also Notepad search with Bing.
Search Previews now support apps, documents and others.
Start Tiles folder naming is now an option.
There's also a way to connect your phone to your PC to read text messages, see photos and notifications.
There are also some known issues worth noting, including being stuck at "preparing to install..." It isn't stuck, it just take a long while, and Microsoft suggest it could take up to 30-minuntes to complete.
For the full information on Windows Insider preview build 17666 see the Microsoft blog