Pentagon's little brother may have stopped growing (for the time being?) but it's sure not retarded!
Triangle I is a perfect FREE entry into the world of analog synthesis. Make no mistake, it cannot compete with Pentagon or Cronox, but I have never come across a single free analog softsynth (and I have all of them, trust me:) that offered more features, a more beautiful, lush sound, or a more logical interface. In fact, it seems like most people (including myself) keep using it even *after* they bought PI, which alone is a testament to the rich - and unique - quality of its sound.
Personally, I am not in love with its look, but it looks like René just might update its GUI to that of Pentagon. Its only serious limitation is that it's monophonic, but nothing stops you from using several of them simultaneously. In all, Triangle I is complex and control-rich enough to give you a fascinating opportunity to learn analog sound creation, but simple enough so that you don't get overwhelmed by all the knobs and buttons (of which, BTW, there are 55 - more than the Roland JP8080 has...)
Many people mentioned the lack of documentation. FALSE! THERE IS A COMPREHENSIVE MANUAL AVAILABLE IN HTML, and while I don't know how you can access it from rgcAudio's website (it seems like René is so busy making his superb products even better, he tends to forget about the website...:) if you send him an e-mail he will provide you with the link.
Summary: if you don't download it and give it a try, you are a fool - and I do stand by that statement.
...but Triangle I is a bit of a different animal... I'm glad I have it and I wish rgc:audio wuz still doin' it to the vsti and mini-host world. Does anyone know if they're developing under a new handle??
No. All this series (Triangle, Square, and even Pentagone) is being totally abandoned for years and years.
Like you I have all the versions of all these wonderful synths, and yes they were a bit different in their results from versions to version... But apart very few exceptions I always use the last version of these old excellent synths created by René G. Ceballos (rgcaudio, rgc was the three initials of his name).