The UNIX Support Group (USG) at Bell Labs took 6th Edition UNIX and made production releases for the entire laboratory, in parallel with Dennis Ritchie, Ken Thompson and the rest of the folks who wrote UNIX originally. In my opinion, the USG guys got some stuff right, but most stuff wrong, and generally were a bad thing for UNIX's overall evolution.
The real trouble started when AT&T agreed to divestiture in January 1984. That's when they decided that UNIX would be the ideal vehicle to try and sell the 3B hardware into the general computing market. AT&T blessed the USG folks as the canonical source of commercial UNIX, and the rest is history.
The market figured out before long that AT&T's 3B hardware wasn't really so hot, and eventually AT&T exited both the systems market (by buying National Cash Register (NCR), and handing over the AT&T Data Systems Division to NCR to manage), and the UNIX market (by separately incorporating USG into UNIX Systems Laboratories (USL), and subsequently selling USL to Novell). More recently, Novell figured out that it had no clue what to do with UNIX, so they sold USL yet again, to the Santa Cruz Operation, which has been a UNIX vendor for some time - they provided Microsoft with the "Xenix" variant of UNIX (and the less said about that mess, the better). Where this leaves things now is anyone's guess.
In my opinion, the "free" UNIX efforts are the salvation of UNIX, because they are operating much like the old U.C. Berkeley Computer Systems Research Group (CSRG); that is, they are evolving the system, picking carefully what things to include in their distributions, and what to exclude. My personal favorite is NetBSD because it is closest to the spirit of the BSD UNIX, and they are paying the most attention to machine independence and software portability. If you code for NetBSD, it's exactly the same on all systems - same include files, same programs, and so on, without regard to what processor or system architecture that you're on. As UNIX was meant to be.
These guys either started from scratch (unwilling to pay AT&T for a source & binary distribution license), or started with real UNIX and then mutated it so much that it really can't be called UNIX any more (e.g. AIX).
[Apple] [Cray] [DEC] [Sun] [SGI]