Single-Protocol Instant Messengers
The instant messaging clients in this section only handle a single IM protocol, and are therefore generally more lightweight than their multi-protocol brethren from the previous section. Now you may be asking, “why would anyone choose a client that can only handle one protocol when there are several multi-protocol options available for free?” Good question! This section is going to appeal to people who have a limited need for IM, or have legacy hardware and need all the free system resources they can get. Single-protocol clients also make sense for those who live in countries/regions where almost everyone is using the same protocol. Since there is no point in comparing different IM protocols, I have listed each client under a sub-heading for the protocol it uses, descending by the protocol's popularity.
WLM (formerly MSN) Protocol
Emesene (v. 1.0.1)
Emesene is essentially a clone of the MSN Messenger client and--you guessed it--uses the Windows Live Messenger (WLM) IM protocol. Emesene is basically the same as the old MSN Messenger. Remove the four instances of the letter 'e,' and you're left with 'msn'.
KMess (v. 2.0)
KMess is a WLM (MSN) client for KDE. One feature that sets KMess apart from Emesene (which endeavors to be a straight clone) is the ability to 'nudge' (like a Facebook 'poke') your contacts to see if they are really away. You also have the ability to send files to your contacts over the IM protocol.
Psi (v. 0.12)
Being available on all three major platforms, as well as having a simple, user-friendly interface, put Psi on the top of this list. Psi easily has the slickest interface of the Jabber IM clients and a right-click on it's system tray icon will present most of it's options in a drop-down menu. Note that it was written in qt, which makes it great for KDE-users.
Gajim (v. 0.12.1)
This is the lightest and fastest jabber IM client. Setting up an account was not a problem for Gajim, and everything is easily accessible via a right-click on the system tray icon. This app was written in GTK, so If you use GNOME, Gajim is a great choice.
Gossip (v. 0.31)
I can only recommend using this app if you have a Jabber account from the official, jabber.org domain. While I was able to chat with contacts on other domains, connection wasn't possible. Gossip was unable to connect using a user ID from jabber.com. After much frustration, I retried using an ID from jabber.org, which connected immediately. Gajim is an all-around better option.
qutIM (v. 0.1)
This app does a good job of cloning the old ICQ client. qutIM even opens in a full-length, thin window on the right side of the screen, which gets irritating. However, it is light and quick. I had no problem getting up and running in qutIM, even though I gave up on ICQ before the turn of the millenium.