Briefly, Plan 9 from Bell Labs is a computer operating system designed by the original UNIX design team, after decades of work on Research UNIX in the late 80's, the team decided to write a new operating system from scratch, Plan 9 was finally released in 1992 (Desktop).
These systems, and variations thereof, have more or less been in continual development since. The history and design philosophy behind these operating systems, is interesting, but we will not talk about that here. Instead, this article will focus on the practical aspects of using Plan 9 as day-to-day desktop system.
Beware that prior exposure to UNIX is a double-edged sword. There are similar sounding commands and conventions between the two platforms, and Plan 9 does follow the UNIX philosophy (much more so then UNIX in fact). Nevertheless, Plan 9 is not UNIX! It is an operating system written entirely from scratch, backwards compatibility was not a goal. If you expect just another Ubuntu spin-off, you will be very disappointed. In fact, lets be clear here: You will be disappointed, period. Now with that disclaimer out of the way, lets have some fun!
In 2002 the 4th edition of Plan 9 was released, it was essentially a rolling release, that continued to receive updated from Bell Labs until 2015, when the project was officially discontinued. In mid 2021 though, Bell Labs gave ownership of all previous Plan 9 sources to the Plan 9 foundation. The goal of this foundation is to continue the development of Plan 9, but so far, not much has happened. There are several community forks around though, two of them, 9legacy and 9front, sprang into existence around 2010. If you want to use Plan 9 as a day-to-day desktop, which will be the focus of this article, I strongly recommend going with 9front.
It is likely the only candidate that will actually run on your physical hardware, and it has many features that a modern user takes for granted, such as auto-mounting USB sticks, wifi support, working audio, video playback and git. 9front has an excellent fqa and community wiki, that do a far better job of presenting accurate information then I do (be prepared for quirky humor though!). Still, it can be interesting to play with 9legacy too, if only for historical curiosity, so I will give some pointers in this article on "classic Plan 9" (9legacy and the old 4th edition of Plan 9 are nearly identical), where it differs significantly from 9front. For classic Plan 9, the Plan 9 wiki from Bell Labs website is a better source of documentation then the 9front resources.