Windows Live Messenger (or MSN Messenger 8.0 if you don't give a damn about the Live brand) is an invite-only beta. You can get an invitation from http://ideas.live.com.After you download the whooping 13.5MB, you get the translucent dialog from the right. There aren't many changes in Windows Live Messenger.
Sharing Folders - new and simple way to share personal photos, documents, and files with your Messenger contacts. You will be able to create a Sharing Folder by simply dragging files onto a contact name in Messenger. This will create an exact copy of the files on both you and your contact's computers, and create a Sharing Folder that can be accessed from Messenger, and on the desktop.
Live Call - using Windows Live Call in Messenger, you’ll be able to start making calls within minutes to your friends across the country or around the world by signing up for MCI Web Calling. You will be able to purchase minutes upfront from MCI and then add minutes when your balance runs low. This feature is not included yet.
If you still wonder what's the Live thing, here's a semi-official explanation: "After a few successful years, the creators of the TV show Buffy the Vampire Slayer split the cast and concept in half and created a new and almost equally successful show, Angel. Buffy focused on the human experience, while Angel catered to the sci-fi-action-loving crowd. Think of MSN like Buffy and Windows Live like the Angel spin-off: same people and same missions, but divided in the interest of better focus. MSN now means content, like MSN.com and MSNBC. While Windows Live means services like Live.com, Mail, and of course, Instant Messaging."