The best feature is it's highlighting. We don't save bookmarks in our minds, but specific sentences or paragraphs. Diigo lets us highlight those and saves them right into my Diigo account/stream. I can then simply search diigo not only by keyword or bookmark, but the phrases that stuck out to me in the first place to make me want to bookmark it.
The second best feature is being able to publish the links to multiple groups so effortlessly.
- http://www.fogbugz.com (hosted) - keeps my consulting and product dev flow going.
- http://www.freshbooks.com - Trivializes the slippery slope of managing billable hours not only for you but for sub-contractors.
- http://www.bitbucket.org - Free, unlimited private repositories. Beneficial to someone like me who has a lot of small projects. I hope Github gets this soon.