Instant messaging is the precursor to the absurdly popular SMS text messaging on cell phones. IM still has a strong demand on the desktop, despite the rise of VoIP.
IM, as we know it today, got its start from AOL Instant Messenger and ICQ in the late 90s. Since then, Microsoft, Yahoo, Google, and many others have entered the fray with their own messenger clients and protocols. Because most people will want to communicate with others, regardless of which IM protocol they use, this is a listing of only those IM clients that can handle multiple protocols. In the Windows sandbox, third-party freeware favorite Trillian is the single industry-leading app to look at for comparison. If, for some reason, you want to install a different client for each protocol or only want to communicate with friends who use your favorite protocol, skip ahead to the next section.
Pidgin is one of the most popular multi-protocol IM clients on the scene today. It includes support for the most popular protocols, such as MSN, Yahoo, AIM, ICQ, IRC, GoogleTalk, MySpace, and many more less-popular options. Pidgin also functions as a top-notch email notification applet in that area of the taskbar. It will alert you to any new emails sent to the addresses corresponding to the IM protocols in use (Hotmail for MSN Messenger or Yahoo! mail for Yahoo! Messenger, for example).
What puts Pidgin above the rest is its availability on all three major operating systems, making it the closest thing to a truly universal IM client available today. My one beef with Pidgin is the inability to close from the notification area icon. Instead, you must first maximize Pidgin from the notification area, and then select Quit from Buddies in the menu bar. Pidgin is currently the default instant messenging client for Ubuntu and Fedora, though Canonical (the company behind Ubuntu) has recently announced that they will be replacing Pidgin with Empathy for version 9.10. It's my guess that you'll see a massive jump in downloads from the Pidgin Web site and in the usage statistics from the Ubuntu repos when that happens.
Kopete is the default IM client for the KDE desktop and many distros that use it, such as openSUSE and Kubuntu.
Like Empathy, Kopete does not handle the IRC or MySpace protocols, but does support Jabber. SMS is an option in the account configuration menu. However, it requires additional plugins and a simple Google search for “Kopete SMS” yielded only out-of-date or downright useless information. Like most KDE apps, if you're running KDE, Kopete will be a better choice than if you run GNOME. For the time being, Pidgin is an all-around better IM client regardless of your GUI.
The version of Empathy that I tested was very much like a Pidgin-lite. It has a strikingly similar appearance, yet doesn't support as many protocols. The one major protocol that Empathy supports and Pidgin doesn't is the increaasingly-popular Jabber. On the flip-side, there is no MySpace or IRC support.
While Empathy's home page advertises Facebook support, a closer examination reveals that it requires a simple hack to actually work. From my experience so far, Empathy is one to watch, but not ready to replace Pidgin as your primary IM client.
SIM (Simple Instant Messenger) is simple, as advertised, and highly functional as well. But it does have one crippling flaw: the text for your contacts is white. This causes your contact's names to blend into the white background, making distinguishing between your contacts impossible, and that's kind of a big deal. No amount of tweaking in the Setup/Interface options fixed this issue.
Only by hovering the mouse over the blank space for each contact could you determine who was who.
Upon further inspection, I realized that SIM was meant for KDE and not Ubuntu's default GNOME GUI. Unfortunately, running SIM in Kubuntu did not fix this issue, therefore I must recommend you take a pass on this otherwise excellent IM client.