http://www.mercury.to/index.php?sub=WhatIsMercury
Seems like a worthwhile replacement to MSN Messenger. I haven't tried it yet, but probably will.
http://www.microsoft.com/mac/
Link to it is on the left. I post this link because it's the main place to go for MS Mac stuff.
As noted, it's not a functional replacement for the Windows version. I use it because lots of friends of mine use the Windows version. I haven't tried to find something that would allow me to see all of them and use something that has video/voice (probably Yahoo would). If you
need video/voice add on, then use a different product. It's a bit surprising that MS didn't add this functionality, as their own Mac Business Unit uses all these products daily. There must be a technical reason underlying the product (like a choice by MS to not support some video codec (?) that Apple builds into the MacOS, or vice versa.