Elastix = Dead/Bought by 3CX. Also was a fork of FreePPBX
PBX In a Flash = Dead/Bought by 3CX. Also was a fork of FreePBX
Trixbox = WAAAAY Dead. Also was a fork of FreePBX
IncrediblePBX = Still around-ish. Offshoot of PBX In a Flash. Was forked from FreePBX
I’ve been using FreePBX for almost 8-10 years, it was around before that. This is the longest slow and painful death I’ve ever seen.