www.octgn.net
This has come a long way and is highly polished by now. The functionality exceeds cockatrice by far, as far as I understand even matchmaking is possible (or in progress of being added) and the deck editor is great.
Cons would be the .net 4 requirement and the need to download and include the setfiles/pics manually.
http://octgn.gamersjudgement.com/viewforum.php?f=8
Hopefully this will keep the online HL alive.
Cockatrice still works, you need to enter play.woogerworks.com as the chosen host.
(http://i7.aijaa.com/b/00780/11900220.jpg)
The program itself is probably better than cockatrice, though.
I test OCTGN right now, it feels much more better.
But one big disadvantage is you can't rejoin a disconnected gamestate... or anyone can disprove that?
Sorry, not supported as of yet.
Quote from: #octgn@irc.freenode.net
17:51:02 < pyyhttu> Hi guys. Tried the client again after a hiatus and I like what I see so far.
17:51:30 < pyyhttu> One question: Does the client support the rejoin after a severed network connection?
17:51:52 < pyyhttu> (So that the game state is able to continue from where it left?)
17:52:10 < db0_at_work> @pyyhttu nope
17:52:52 < pyyhttu> db0_at_work: Ok, thanks. I didn't take a look at the issue tracker yet, but do you know if this has been requested by
people?
17:53:14 < db0_at_work> it has
17:56:42 < pyyhttu> Yeah, found now. https://github.com/kellyelton/OCTGN/issues/43
17:56:45 < pyyhttu> OK, thanks.
I would recommend sticking with cockatrice for the time being and their alternative servers.