Currently, five prototype features are available through Facebook Prototypes:
Desktop Notifications is a widget for Mac OS X which adds notifications about Facebook goings on to your desktop.
Similar Posts adds a link to posts on your friend feed allowing you to see similar content with one click.
Enhanced Event Emails includes an iCal file as an attachment with event alert emails sent by Facebook. Click it and it'll add the event to your iCal.
Photo Tag Search implements a new search area where you can scour tagged photos. It'll let you search for photos specific friends have tagged and particular tags.
Recent Comment Filter is another pretty self-explanatory one. Select it on your news feed and you'll see the latest stream of comments from your friends.
Take a look and try the new features for yourself. We'd really like to hear what you think of Facebook's experiments.