Yahoo Messenger 9 Beta has a Vista’ish interface,flickr integration. Video and images can be viewed right inside the messenger window. Y! 9 Beta allows message/call forwarding, making it a little better SIP client. A desirable feature would have been to allow a user to sign-in into the IM account from multiple places/devices eg. Mobile Phone, Office Computer. Google lets users sign-in into IM service from multiple locations.
Check out video http://beta.messenger.yahoo.com/video.php
GoogleTalk is well integrated with Gmail, Orkut and it also has a voice chat facility.
MSN Messenger (now Live Messenger) is well integrated into MS Windows and in a typical MS way introduces in all the programs. For eg. you can’t terminate MSN Messenger when any of the MS Office applications are running.
Pidgin has a lot of improvements over Gaim including better support for IM contacts on yahoo. I prefer multi-protocol clients since they let me connect to multiple IM accounts simultaneously. However a significant missing feature is integration of voice chat features.
I hope all IM networks will be able to connect to each other, instead of being islands and yahoo IM users would be able to talk to Google or Skype or any other IM service via test/voice/video.