There's the new version of messenger for Mac. Version 7 but it's still far far away from the Windows live messenger version.
They added audio/video chat, but only for corporate accounts, not for personal accounts! So pretty useless for the normal home user. (but I guess we will get that feature later; I haven't really used it anyway when I was on XP, because the quality wasn't that good. Skype did a way better job in my eyes)
Otherwise they added a search bar for your contacts (I am not using that either because I don't have too many contacts)
I've read somewhere they added file sending, so that might be useful.
Personally I think the new dock icon is worse than the one from version 6. I just changed it back to the old one myself. Otherwise I like the new design (for example when someone comes online, you get the info pop up in the lower right corner whicht looks nicer than before)
Features I still miss:
-When you click on a buddy name in the live version, then you see his business card from where you could go to the msn online spaces of others which I always found useful. I think you are not even able to go to the msn spaces through any menu option anywhere in the Mac version. So you have to know the exect web address yourself. And I don't always want to check if someone changed some info there.
-Nice would also be if you could change your status just where it is (with a drop down menu like in the Windows version) instead going throw the "network" menu in the menubar. It annoys me taking 3 steps instead just one drop down menu.
-Tabbed windows (won't see that in near future I guess, even the Windows version doesn't have it. If I should really miss that in the future, I will give Admium another try.)
I also found a first small bug: instead "in a call" you will see a second "out to lunch" choice in the status message choices. (how does such a spell error get in there? they shouldn't had changed anything there)
The two versions will never be the same because there are two completly different teams working on it. At least we can chat comfortably with our Windows buddies.
Blog from the developper team about A/V:
http://blogs.msdn.com/macmojo/archive/2007/11/12/a-chat-about-messenger.aspx
http://blogs.msdn.com/macmojo/archive/2006/09/27/774149.aspx