Esf 1.2.3 is the last stable release and the one you will have greater chances of finding people still playing. Open Beta Final is just that, its a long outdated version of Esf which was supposedly put out so it would be easier to squash bugs. It didn't work out though.
Now comes Esf: Final. It is the version the Esf team is currently working on and it will be the final Esf release according to the devs. All the current pics and videos are of this version. And like us you will have to wait for it to get released.
Now if you have Half-Life attached to your Steam account, i suggest you go
here and get it validated. After that feel free to take part in the community and have a great day.