AOL Instant Messenger, better known as simply AIM, is my preferred way of instant messaging simply due to the fact it’s the most reliable chat network. Out of all the things AOL does wrong, AIM is the only thing they do right.
Windows Live (formerly MSN) and Yahoo both have routine small outages on their networks that can get really irritating really quickly – but this doesn’t happen on AIM. Ever.
And for those that say "What about Google Talk?", yes, GTalk’s Jabber way of doing things is extremely reliable but unfortunately doesn’t have as much of an established presence like AIM does.
AIM Lite is an AIM chat client released the AIM team themselves. It is available for both Windows and Mac. AIM Lite one of the absolute fastest and lightest graphical clients I’ve ever used. The fastest I’ve ever used was Miranda, but AIM Lite comes darn close.
For those who are all about speed (or if you just have a slower computer) and use AIM, I strongly recommend getting AIM Lite.
If you like the software, see the plugins available. These plugins add in features like sound notification, toaster pop-ups (the good kind of pop-up) and a few other options.
