- Public folder - All files in there get a unique URL for publishing on the web
- Shared folders - You can share folders with your friends. Of yours they have to have a dropbox account
- Versioning - All files are automatically versioned. It's like a simple version control system which may be great if you collaborate with your friends on documents, but also if you just want to restore an older version because you broke something in the newer one. If you collaborate on word-processing documents I recommend however to try GDocs which has an real-time collaboration feature and is now even accessible offline.
As I said...online file sharing and sychronization becomes more and more popular. Here's the Microsoft solution:
Here's also a video, unfortunately only in German: