Pidgin (ex-gaim), as it is the main and the most used IM client for gnome nowadays I'll start talking about it. It's supports tons of protocols like Msn, Irc, Jabber, Yahoo!, AIM and lots of others (and thats one of the reasons that i don't use it, since I only use msn protocol). It has a very simple UI and has full support for Display Pictures and File Transfers.
The Chat window is very clean, the DP of who you talking is only displayed s very small icon on the top-right of the screen, this is causing lots of discussions between the developers and users. I really prefer the mock up done here that uses the DP in the normal size, but other people think that it would be lost of space.
emesene is a Python based msn client. It's aimed to be as similar as possible to the official client but using a simple and nice look. I really like the idea of interface that emesene uses, It gives importance to the display pictures the size it was ment to be seen.
It support Personal messages(what is not very easy to se in linux msn clients) and It has full suport to Diplay Pictures, I've tested and it can send and recieve DP's from all type of clients, but saddly it doesn't support file transfers yet. I think its a very good client, mainly for people who got used to windows msn messenger.
That's it for now, next time i'll talk about Monkey Messenger and Galaxium.