Is there a list of all available extensions for Jupyterlab?

You can see a list of jupyterlab extensions by browsing the jupyterlab-extension topic on github (as suggested by the jupyterlab docs):

https://github.com/topics/jupyterlab-extension

Found in Jupyterlab tutorial Documentation -> Release 0.1 -> Chapter 2.2 -> Installation extension.

The currently installed extensions can be listed by running the command: jupyter labextension list.