I had Trillian but I hated it, I have Trillian Pro with loads of plugins to do things, yet it just didn't look nice. I hated that it would not really show the difference between the services it offered.
It also used to pop-up when I was typing somewhere else and I'd send the wrong text to the wrong person (Don't you just hate that?).
I tried Y! and it's alright but I didn't like the way it signs in. MSN Messenger is teh evil if you ask me. It's full of people who tipe liek dis and I can't understand them. GTalk looked like it had potential it was light and easy to use with great VOIP support, but who uses it?
Skype is good though, I like Skype I have a VOIP handset for it, but I recently added bluetooth support for it, and not I can talk wirelessly and at 1.2p a minute to almost any country in the World I can't argue.
I tried ICQ but it's so complicated, I mean how big are those numbers? I can't be bothered to try and rember one. Jabber? I thought Jabber was the protocol and not a specific messenger program, but I may be mistaken.
AIM is even worse that MSN from what I can tell the only people who use it properly are AOL users and that should be reason enough not to install it, but I have once used it and it seems you can register an AOL account that only works for AIM and can't use it for anything (a thing I like with the MS .NET Passport is that you can use one login for everything).
I have ChatZilla aswell for IRC channels, but as I don't use IRC if I can avoid it because it tends to be full of weirdos.
So installed now I have GTalk, MSN Messenger, Skype and ChatZilla.