BookmarkG is a new bookmarking service that aims to fill the gap left by mainstream bookmarking utilities by focusing on groups. Users can create their own group, invite friends, and begin adding links with tags, ratings, and metadata that can be searched.
What makes the product interesting is that you can set the invites to be open, so once people are invited they can invite friends of their own--similar to Google's invite system in Docs and Spreadsheets.
Where I think BookmarkG is going to run into trouble is in ease of use. To me the most important aspect of any modern day bookmarking site is the browser integration, and BookmarkG has none of it. You can't import existing bookmarks, and there's no extension or bookmarklet to let you add bookmarks on the fly. You'll have to remember to go back to BookmarkG from whatever site you're on, then copy and paste in its information manually. Compare that to something like Delicious, where it can pull in related user tags and images and it looks like old hat.