IRC Chat

Pros:
Lag free
Allows multiple options for channel owners and chatters (can be made invite only, limited amount of users, etc)
Allows the creation of new channels (if you have a faction etc, you can use this when your server may be down or when you do not wish to play minecraft)
Can be coded
Allows for multiple different op levels (Owner,Full op,Half-op)
If you own an IRC server there are multiple other options for server admins, server operators, server bots, etc.
Can be accessed by Iphone/Ipad, Android phones and tablet pcs.
Easily moderated without any extra coding/effort
Allows for quick channel lockdown (mutes everyone except mods if there is a spam attack)

Cons:
Is susceptible to botnets (if the IRC server has no protection against this)
Can be spammed (Easy to script a spam preventer into a bot)
has no icons (is a pro in my opinion but con in others)
Does not have noise when someone posts a message (If you are using a client it may be different)