Community Engine is a free, open-source social network plugin for Ruby on Rails applications. Drop it into your new or existing application, and you’ll instantly have all the features of a basic community site.

Features of this social network plugin:

• Authentication (sign up, log in)
• User profiles
• User search
• Blogs with tagging, categories and rich text editing
• Photo uploading and tagging
• “Clippings” – visual bookmarks
• Commenting and comment notification by e-mail (Blogs, profiles, photos, and clippings can be commented)
• Forums
• Friendships and activity feeds
• Favoriting of clippings and blog posts
• And more!

Attractive social bookmarking icons are a factor in website visitors clicking on them and help everyone to share, bookmark & promote your content.

WebResourcesDepot have collected these beautiful social bookmarking icon sets: 

http://www.snap2objects.com/2007/11/06/free-vector-social-bookmark-icons/
Package includes PDF vector format and PNG social bookmark icons.

http://vikiworks.com/2007/06/15/social-bookmark-iconset/
Package includes 1 PNG file with all social bookmark icons.

http://vikiworks.com/2007/07/28/social-bookmark-iconset-part-2/
Package includes 1 PNG file with all social bookmark icons.

http://fasticon.com/freeware/?p=65
Package includes 10 social media icons.

http://fasticon.com/freeware/?p=72
Package includes 17 bookmark icons.

http://www.freakgroup.com.ar/pack-de-iconos-sociales-v10/
Package includes 12 social bookmark icons in PNG format.

http://utombox.com/my-works/

Standard 16×16 GIF social bookmark icons.
http://www.feedforall.com/social-bookmarking-icons.htm