I think Channel Chat would be a good plugin it allows you to split the chat up into channels.
http://dev.bukkit.org/server-mods/channel-chat/
Example:
Say you have 5 people chatting about something they did on the weekend and you have another 3 building something together.
You want to split the chat so only the 3 building will see what each other say and the 5 talking about the weekend to only see what they are talking about.
