Okay, a very low-effort post today...!
There was a recent Reddit thread on universal RPG systems, so I had to chime in and spread the love for the D6 System, my current mechanical obsession. Another poster asked if I could explain the difference in OpenD6 and Mini Six (as well as TinyD6), and I ended up writing a bit more than I imagined I would. Then I realized that there's a chance other people might google something like "OpenD6 vs. Mini Six," and that it might be helpful to have that same response as a blog post, especially for anyone who avoids Reddit threads when they search.
TinyD6 is the one I know the least about and have never played. As far as I know, it is its own system using one or more (I really don't know which) d6s and is pretty rules-lite. It seems to have a following of its own, and content in a variety of genres, so it is probably a good gateway to "univeral gaming" even though...AFAIK...it is not directly related to the other two.
OpenD6 and Mini Six, on the other hand, are very much connected. OpenD6 is the license/movement/set of games that came about when the owner of West End Games' D6 system (used in Star Wars and DC Universe, with a proto-version in Ghostbusters, among others) released it via the OGL. You can probably find a lot of the old WEG books...or versions of them with IP removed...for free on the net, although it's been a while since I've surveyed that availability. At its heart, the D6 System is more of a toolkit than a specific way to play, and there's even an old book simply called The D6 System (with the subtitle The Customizable Roleplaying Game) that is often called "the cookbook" and has a bunch of options laid out with the idea that you'll choose the ones most appropriate to your game (including stuff like which attributes you'll have for characters). There was a set of three books for Fantasy, Adventure, and Space that had rules specifically tailored to those genres. (Space, for example, was built heavily upon the old Star Wars rules.)
Once OpenD6 became a thing, the Mini Six system was one of the first to use the license. It is a very approachable, fairly stripped-down ruleset that is also available for free and that can be used for just about any genre. At the time it was released, I saw it as the authors (Ray Nolan and Phil Morris of AntiPaladin games) sort of reverse-engineering Ghostbusters (probably my favorite RPG ever) while bringing in some of the options and technology that had been innovated for the D6 System over its lifetime. I definitely recommend checking it out:
Overall, the potentially diverse application of the system was a plus for folks who really like to tweak their games, but some of the drastic differences in how the games end up playing out probably made it tough to gather as much of a unified community as there was for, say, the d20 System. It's typically pretty easy to convert from one "subsystem" to another, but I just don't think that feels as universal as systems that have a unified set of attributes (and skills, and even how to read the results of rolls, in the case of games like DC Universe that used a success/fail-based version called D6 Legend).
The D6 System has recently gotten some love with stuff like the Zorro RPG and a recent Kickstarter for a "D6 2e." I've gravitated to my own houseruled version as a sort of house system for my own games, and so I check out new developments when I think about it and have the time, in case there's something I can bolt onto the form of the game I'm running with.
And then I followed up quickly after to add this:
Also, just wanted to add that the owner of the system IP who released it into the wild was Eric Gibson of Purgatory Publishing. It was a really kind move that definitely helped keep the system alive. There are a few other folks who have been vital to it over the years, but I don't want to start naming them because I'm afraid I'll leave out someone important...!
If you happen upon this while investigating the system(s), please let me know if there's anything more to my opinions that may be helpful in your search!