If you ever run into some problems on your PC with the messaging softwares of different vendors such as Windows Live Messenger, Yahoo Messenger, Gtalk , AIM etc… then i would recommend using Meebo.

Meebo is a browser based messenger and allows you to connect to WL Messenger, Yahoo Messenger, AIM… at the same time without using their individual respective softwares, which is really cool. This is one of the most realistic and simplistic browser based messenger i’ve seen and is pretty quick too.

Of course, you won’t have all the advanced features such as voice-calls but still it comes in pretty handy for me especially when I’ve problems with signing into the vendor softwares on the PC.

Note: This is way better than the online MSN messenger.

Link: Meebo

Update: Kool IM is a simillar site suggested by SamerBazzi, it adds support for other services like Jabber and Sametime. This service though, requires pop-up blocking to be disabled for the site so that it can properly.