Diablo III Unofficial IRC Channel -> #Diablo3

Server: quakenet.org
Channel: #diablo3
Current users count as of writing of this post: ~850-900
You can find a list of all available servers of quakenet here-> http://www.quakenet.org/servers

If you use an mIRC or X-chat you can just type in to the console
/server clanserver4u2.de.quakenet.org
(after you connect)
/join #diablo 3

Alternatively you can head over to their WebIRC Chat and manually type in username and channel.

How to check the status of the Diablo III Servers and Realms

You can check here which Diablo 3 game and auction house (AH) servers (realms) are online or offline ->

http://eu.battle.net/d3/en/status

This page lists all available Diablo III game and auction house servers, as well as the status of each -- either available or undergoing maintenance.

            

PS: Note, this is for the Official Blizzard realms (a.k.a. retail srvs), not a private server (prv srv).
If you seek a diablo 3 prv srv check some Mooege repack or compile it on your own (check this blog on how to do it).