On December 15, 2017, some six months after killing off AIM multi-user chat, Verizon/Oath took the entire AIM service out behind the barn and put a bullet in its head. It's dead, kaput, finis, gone, terminated with extreme prejudice. It has ceased to be. It is no more. Bereft of life, it rests in peace. It has rung up the heavenly curtain and joined the choir invisible. It is an ex-service.
It will not be replaced by anything. It is just dead as a doornail. Period.
But Barflies are nothing if not resourceful! Pending the more complete alternative Scott Bragg is working on, there is an interim replacement bar chat powered by ejabberd running at jabber.caerllewys.net. This server is now fully secured with a 4Kbit SSL certificate.
You will need an XMPP-capable client such as Pidgin, Adium or Miranda, and an XMPP account to connect to it. I have now verified that you should be able to connect to chat.caerllewys.net using ANY valid XMPP account from a properly configured service — Google Talk, for instance. (Not, however, LJTalk. I'm told LiveJournal did something on their servers that makes LJTalk accounts unable to connect to non-LiveJournal XMPP servers. Presumably they accidentally, or maybe intentionally, broke federation.)
XMPP.org has a list of XMPP clients here. There is also a very lightweight pure-Java client at JBother.org, which allegedly should run on anything with Java 1.4.x or later installed. If you're using Miranda, Douglas King has put together a complete tutorial here (Flash required, unfortunately).
(Some clients, Trillian v2.x for example, may call the protocol Jabber instead of XMPP. But it's the same thing. Jabber is just an older name for it.)
Again, these example instructions are for Pidgin, but other XMPP clients should be generally similar. The example at right shows connecting with Pidgin using a Google Talk account.
If you don't already have an XMPP account, or for some reason you cannot connect with the account you have, you can register a new account at jabber.caerllewys.net in any XMPP-enabled client. This example is for Pidgin; Adium should be almost exactly the same; other clients should be generally similar.
The Trillian online manual has instructions here for setting up a Jabber/XMPP account. That manual is apparently for Trillian 2.x; Trillian 3.x should presumably be similar.
Pidgin 2.6.1 on Windows appears to have a bug that affects creating XMPP accounts. If you use Pidgin on Windows, it is older than 2.6.2, and you don't have an existing XMPP account that you can use, consider upgrading Pidgin and rebooting Windows before trying to create one. (If you don't reboot, the old Pidgin DLLs will still be in memory, and will screw you up, because Windows is too stupid to reload the newer versions without a reboot.)
If you're using bitlbee with its built-in jabber support, try the following set of commands:
chat add [YOUR ACCOUNT ID] firstname.lastname@example.org
If you're using bitlbee and libpurple, you need this additional command FIRST:
acc jabber set connect_server talk.google.com
(Thanks to Kent Ritchie for the above.)
If you're still using Gaim, it does not support XMPP at all. In this case I recommend you first upgrade to Pidgin, start Pidgin once and close it, then — BEFORE you set up your XMPP account — find your Gaim and Pidgin preferences directories (they will be named .gaim and .purple) and copy the two files accounts.xml and blist.xml from the .gaim directory to the .purple directory. These files contain all of your existing account and buddy information, and copying them over (and making sure you can connect to your existing accounts from Pidgin) BEFORE you add any new accounts will save you a lot of trouble later.
If you have trouble reconnecting to your AIM account, go to the account list, Modify the AIM account, go to the Advanced tab, and uncheck 'Use SSL' and 'Use clientLogin' if they are checked.