For those who like real-time chat, there's a chat server here. Quick instructions: point your irc client at the server irc.slashnet.org and join channel #writing, or head over to the java chat page (see below).

There are a couple of scheduled chats, co-hosted by Toasted Cheese

As far as irc clients go, from what I hear mIRC is one of the best for windows. For the mac, there is Ircle, but I haven't heard anything about it.(It was just on the first page of results google returned.) I use a program called irssi, which runs on windows, mac OS X, and most unixes.

Full instructions:

1. Get an IRC client - Depending on the operating system you run, you have different choices available for an IRC client. Some clients are easier to use than others. You can search google for irc clients for your computer if you don't want to use the ones I linked to above.
2. Setup and install the IRC client - Follow the instructions that come with the client. Read the readmes. They're all different, so I can't help you here.
3. Connect to irc.slashnet.org - Once you have installed and setup your IRC client, connect to irc.slashnet.org. In a GUI program like mIRC, you can enter this server name in the connection box. In most IRC programs, you can type "/server irc.slashnet.org" in the text area (without quotes, of course).
4. Join #writing - Once you have successfully connected to the server, join the channel (or "room") #writing. In a GUI client like mIRC, you can do this via a dialog. In most clients, you can type "/join #writing" in the text area (without quotes). Don't forget to press enter after every command.

Java Chat

If, for some reason, you absolutely can't install a proper irc client (say, you're at work), you can use the java client. If you can install a real client though, it's much better. The web just isn't suited for this kind of stuff, and real clients are so much easier to use and more powerful.

Either way, just pick a nickname (any one you want - the only time it'll refuse you is if someone's already using that name at the instant you request it. You can also change nicknames partway through by typing "/nick newnickname") and join the chat. You won't need to change any of the other required fields, so leave them alone.