Last updated March 22, 2012.
Using IRC
The fastest and simplest way to access freenode irc is via http://webchat.freenode.net. If you are connecting through an app, it's on irc.freenode.net.
To learn more about how to actually get connected to IRC, there is a good write up on finding a good client and connecting to IRC at http://groups.drupal.org/node/2326. You can also watch videos about connecting to IRC (note to IRC newbies, the video references a tool, "mibbit" which has been blocked by Freenode, so don't try to use it). The intention of this book page is how to interact once you get connected to IRC.
There are a few simple IRC commands that should be used to allow best use of the IRC channels. To join one of the groups listed on the previous page, type /join #group where #group is name of the group you want to participate in. To disconnect from the server, use /quit.
Register your nick with freenode
When you log into IRC you're going to want to register your name with NickServ so no one else can use your nick, and others can gain familiarity with you by you using the same nick. Learn how to register your nick at http://freenode.net/faq.shtml#nicksetup.
As a courtesy to the Druplicon (see below) and other members of the chat room, please do not include "Drupal" or any variation of it in your name.
Use the same IRC nick that you use with drupal.org
Most people use their Drupal user name as their nick in IRC. If that name is already taken on freenode, then append a dash or numerals to the name. Many IRC clients will automatically append an underscore to your nick upon join if the name is already taken. Having a familiar nick allows for consistency between IRC and the Drupal site.
If you are using a different name on IRC than on Drupal.org, you may wish to use an IRC cloak. Information on obtaining one is here: http://groups.drupal.org/node/5403