Features (From the Microsoft Connect website) :

* Sign-in and messaging in multiple locations - You asked for it, now you have it! You can now sign into Messenger from more than one computer at a time, up to four at once. Simply sign into Messenger from one computer, install Messenger on another computer and sign in using the same account. (Only works if all computers are running Messenger 9.0 Beta. Currently enabled only for users whose Windows Live ID country is US or Canada.)
* Signature sounds – Pick your own personal sound that your contacts will hear when you sign into Messenger (only works if you and your contacts have Messenger 9.0 Beta).
* Per contact sounds - Pick unique sounds for each of your contacts. You will be able to tell from across the room who is sending you a message.
* SPAM Reporting - If users send you unsolicited messages or invitations that you consider spam, you can block them and report them as spammers. If you report a user as a spammer, the messenger service will collect this data and put appropriate restrictions on the spammer accounts. All reports of spammers are kept confidential.
* Animated Display Pictures – Show your moves! Messenger now supports animated .gif files as display pictures.
* Link in Personal Status Message – URLs in Personal Status Messages are hot and clickable in the Messenger contact list.
* Behind the Scenes Changes – Architectural changes that modified how things work but shouldn’t have changed how things look
* Changes to sign-in, connectivity, presence, sending text IM’s, voice & video features
* Windows Live Contacts Server – wlcomm.exe runs in the background to keep your contacts’ information up-to-date and helps Messenger sign you in faster