If you like Messenger on your PC and want the same on your Mac this piece of software rocks.

aMSN is a free open source MSN Messenger clone, with features such as:

» Offline Messaging
» Voice Clips
» A New Login Window & Contact List (As of 0.97)
» Display pictures
» Custom emoticons
» Multi-language support (around 40 languages currently supported)
» Sign in to more than one account at once
» File transfers
» Group support
» Normal, and animated emoticons with sounds
» Chat logs
» Event alarms
» Webcam support
» History in color
» Ability to autoclose the filetransfer window when it is finished
» New plugins log window (Alt-P)
» Plugins compatible with supported languages
» Faster loading skin window
» Skins in chatwindow
» Resizable chatwindow bottom
» When you add a new user, you can define in which group you want to add them
» When a contact change status, the top of the chat window changes color ( green for away, grey for offline, etc.)
» New panel GUI to alert when there's a new aMSN version
» Possibility to download the new release directly by aMSN
» Automatically update language files and plugins from the web
» New gui for deleting users (also possible to block on deletion)
» Possible to add notes for each user (XML)
» Support for MSN Mobile service
» Tabbed windows so you can group conversations using tabs
» Status is displayed inside a frame, so it won't dissapear when you scroll
» Display picture in notification alert (like MSN 7)
» Statistics for history (logging)
» Now possible to choose any state for your login status (before it was only possible to login invisible or online)
» Timestamping


Also available are Plugins and Skins for it.

Check out the :

Screenshots
PlugIns
Skins

Forums for aMSN

I am currently using it and can't say enough good about it.