Essentially, Opera Unite allows you to quickly, easily, and freely share files on your computer, access media, host websites, chat rooms, etc. straight through the browser.
The services advertised on the Opera site:
File Sharing - A simple and safe way to share files directly from your computer.
Photo Sharing - Share your personal photos with friends around the world without the need to upload them.
Fridge - A fun place for people to leave notes on your computer.
The Lounge - Invite your friends to a chat in The Lounge hosted on your computer.
Media Player - Access your complete home music library from wherever you are.
Web Server - Host your Web sites running from your own computer.
They're offering all the tools for you to write your own services (http://dev.opera.com/articles/unite/)
If you hit the jump at the top of the page, you can try out a labs version of the app.
EDIT: Here, home.samuelrsmith.operaunite.com - I've set mine up, haven't added anything to it, yet.
