This has been suggested before
(
http://www.noobscraft.com/suggestions-ideas/online-chat/)
An online chat will require 24/7 moderation(as Adrian stated) , also it may require a lot of memory to use.
You can use Teamspeak to chat on if you wish.
Apart from that, a chat would be nice, maybe something in the PM section which allows you to message a friend and receive the message instantly and not having to refresh 24/7 and some sort of global chat perhaps.
Anyways, Unstable Bukkit should be out this weekend, and maybe the week after Bukkit releases the stable one, then maybe server will be back up