UX is not particularly great nor is the aesthetic, but arguably the most useful tool for personal organization of one's browsing experience. I do not use this service for its social component, but rather for its content management.
Mature and proven way of keeping track of interesting links across the web. I use it more for my own personal organization, than for its social features. This makes it very different from sites like reddit or Digg which are about giving you a feed of content that you'll likely only consume once. Delicious makes it easier to keep track of content that you'll refer back to again and again.