The Windows 11 preview build is affectionately titled Build 22000.51, and Windows Insiders can download it this very second. It contains many of the features shown off during the reveal, but is missing a few bits and pieces. However, Microsoft plan on adding more over the coming months.
So what's in the preview build? Well, you've got the rounded corners, multi-tasking "snap features", and that centered taskbar. You'll find a widgets button on the taskbar too, so you can look at nice boxes which tell you things like the weather and highlight calamitous news stories. The reworked Microsoft Store is also there to peruse, and you'll get to try out the new setup experience. There's plenty I've missed out here too.
Of course, this Windows 11 preview build may hit you with an error or two...or twenty. Things might not open correctly, or stuff might be missing - little things. You can see the full list over on the official blog post.
And I suppose you're wondering, "How do I become a famed Windows 11 Insider?" Well, you register to become one, and blam, the preview build will be yours to test out and give feedback on.
Windows 11 is due out later this year and will be a free upgrade for all Windows 10 users, which is nice.