1. Buddy State Notification

Notifies in a conversation window when a buddy goes or returns from away or idle.

It's useful if you're talking with somebody and he goes away and you want to be notified of his return.

2. History

A must for everyone that need to see past conversations.
To use this plugin you need to enable the Logging option from Tools > Preferences > Logging.

3. Message Notification
Provides a variety of ways of notifying you of unread messages.
It's usefulness is best when combined with the buddy state notification because that plugin inserts a text line in the chat window
that triggers the message notification.

4. Psychic Mode
Causes conversation windows to appear as other users begin to message you. This works for AIM, ICQ, XMPP, Sametime, and Yahoo!
It's useful if you want to make fun of somebody because nobody usually expects you to say something while they start typing to you for the
first time

5. Send button
Adds a Send button to the entry area of the conversation window.
Do you miss the send button from the chat area ? Now you can add it back easily

6. Off-the-Record Messaging
Preserves the privacy of IM communications by providing encryption, authentication, deniability, and perfect forward secrecy.
For people that need to be sure that what they talk with somebody remains strictly between them.

7. Muzic Tracker
The MusicTracker Plugin allows you to customize your status message with information about currently playing song from your music player.
If you want to show your chat buddies what are you listening to.