Source: https://www.baeldung.com/oracle-jdk-vs-openjdk
The following implementations, listed in alphabetical order, are open source and free to use:
AdoptOpenJDK
Amazon Corretto
Azul Zulu
Bck2Brwsr
CACAO
Codename One
DoppioJVM
Eclipse OpenJ9
GraalVM CE
HaikuVM
HotSpot
Jamiga
JamVM
Jelatine JVM
Jikes RVM (Jikes Research Virtual Machine)
JVM.go
leJOS
Maxine
Multi-OS Engine
RopeVM
uJVM
There are also copyrighted implementations:
Azul Zing JVM
CEE-J
Excelsior JET
GraalVM EE
Imsys AB
JamaicaVM (aicas)
JBlend (Aplix)
MicroJvm (IS2T – Industrial Smart Software Technology)
OJVM
PTC Perc
SAP JVM
Waratek CloudVM for Java