I've been using AIM for forever. I even played with MSN Messenger. Both of these clients got so heavily ad-based that they turned me off. And I had to have 2 clients installed and 2 different buddy lists.
Then I came across gaim. Don't go looking for it. It's gone. It has been replaced by pidgin. It's still client-based but consolidates the following protocols:
But it's still client-based so I have to install it everywhere (yes there's a portable variation) and the logs are local to the application instance.
Then I found meebo. It's web-based so there's no install and it consolidates the following protocols:
Closer. And meebo even supports iPhone, Android, BlackBerry and Symbian. But... MSN won't allow but a single logon per identity. This means that if I'm logged on to MSN at one computer using meebo and go to a second computer and logon to MSN using meebo, MSN force logs me off of the first computer. Then if I log off the second computer, I'm not logged on to MSN anywhere. I kept searching.
I finally found imo.im. Yep, web-based and supports the following protocols:
And imo supports iPhone, iPad, Android, BlackBerry, and Symbian.
Here's the twist. imo establishes the sessions with their server, not your computer. Let that soak in. Now your MSN session is with imo's server and you can have multiple concurrent sessions.
You can even go from one computer to another computer and pick up the conversation in progress. Really nice.
imo optionally will maintain centralized logs and even encrypt them. And you can link your identities. What this means is that when you login to imo with one of your identities all of the linked identities will be logged in as well.
Easily the best I've found so far.