It's free and a product by rgcAudio, so you might as well stop reading this and just download it. René's instruments are always musical and of high quality. This is certainly no exception. You could look at it as the little brother of the polyphonic Square I, another of his highly regarded older generation virtual analog synths.
Being a monosynth, some of the most common uses for Triangle is bass and lead sounds, which it handles well. The filter on this little guy has a very warm, musical quality to it that is kind of rare in the virtual world. Though I'm sure it is capable of aggressive sounds, it definitely seems to lean towards sweeter, gentler timbres. The newer engine instruments such as Triangle II and Pentagon I seem better suited for more powerful, edgy, aggressive sounds.
With 3 oscillators, 4 LFOs, built-in effects, more advanced control features like aftertouch sensitivity and various other features, it is surprisingly flexible for a freeware instrument.
Bottom line: It has a unique sound, lovely little musical filter and is free. Definitely a worthwhile download.
...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).