Bookmarking tools for private communities

I've been on the lookout for a tool or site that would allow a private community to share links and discussions. There has been a real need for this among a group at work, as we explore innovative ideas and resources. At the moment we all use different tactics - I use Delicious, there is a share drive folder (ugh), and of course there's the ubiquitous link in an email.

Here are a few tools I've considered over the past year, I would love to hear any other suggestions.

Delicious
I've been using Delicious as a personal bookmarking tool for over a year now (click here to see my collection). It has some great features:

• Integrates with your favourites in Firefox or Flock browsers - if you add to your browser favourites you can also publish it to your Delicious account. You can also set up a button in your links bar that will publish a site directly to Delicious
• Also offers web-based posting of links for when you are unable to access a buttonbar plugin
  
• Fantastic predictive tagging facilities based on your existing personal tags and how other people have tagged it.
• Great search and sort facilities - eg you can browse one tag (such as web2.0) and then refine the list with an extra tag such as "twitter"
  
• Public access - share the links you've tagged eg "web2.0" with other people
• RSS feeds for your account or for a particular tag - so people can automatically receive notifications of any sites you've tagged eg "Web2.0".
  

However, Delicious falls down a bit when it comes to creating a community for sharing links.

• it lacks privacy settings which limits its use as a potential business tool for security reasons
• there is no group function (although an individual can create a network of other individuals). This defeats the concept of a collective library of tags and sites, as they can only be added to an individual's account. Also, I would like to have the facility to add and remove eg team members as they come and go.

Ning
Ning allows for the creation of a private or public community. Have a look at this Library 2.0 community to get a feel for it. The individual community can set up a huge variety of features:

• Welcome page
• Personal profile page
• Forums
• Blog aggregators
• Post links
  
• Post videos and images
• Chat
• And more

Ning is a fantastic tool for creating a community site, but it's not really a bookmarking tool. While it allows you to share links and bookmarks, there is no way to organise and sift through them.

Diigo
I came across this yesterday so I'm still exploring it. However it's looking pretty good:

• Both a personal bookmarking tool and a group tool
• Adds a toolbar to IE, Firefox and Flock for quick bookmarking or post directly to the website
  
• Allows you to highlight and annotate sections of a website that you bookmark
  
• search for a particular tag or view a tag cloud (I don't feel this is quite as functional as Delicious)
  
• Create private or public communities
• Administrator can easily add and remove users

Microsoft Sharepoint
Sharepoint has the advantage of sitting directly behind the firewall, however it lacks the "plug and play" functionality of eg Diigo which is so easy to set up. It also sits outside of the workflow - while both Delicious and Diigo allow you to click a button in your toolbar to bookmark a site, I suspect you would have to do the good old cut-and-paste to add to a list of links in Sharepoint. Also, Sharepoint out-of-the-box functionality is not maximised for easy web 2.0-style browsing and searching - no tag clouds, and essentially no easy way to tag in the first place. It would require extensive modification to make it as friendly as online sites - which means involving IT or developers, and that of course means it's no longer a "lightweight" solution.

I'll be playing with Diigo over the next few weeks and will update you on how it goes. I suspect the main barrier will be whether our firewall allows sufficient access.