Extra features in the new RockMelt browser, backed by Mosaic developer Marc Andreessen, are hidden in the browser frame. Two "wings" to the left and right of the page show who's online (left) and what's new (right). Share buttons and a new search feature are at the top.
The left-hand wing shows you which of your Facebook friends are online. You can read updates or start chats with them from here. You can also filter the list to just a few people you want to see in RockMelt, which remains in beta.
The extra search feature (beyond the search built into the address bar) is very strong. It lets you fly through search results quickly. An "open in new tab" button appears in highlighted results and lets you pop pages into background tabs.
You have to be invited by a RockMelt user to get the beta. But as soon as you sign up for the program with your Facebook ID, all your Facebook friends who use the browser will see that you're waiting to get into the club. Any one of them can invite you.