LinkedIn: I have used LinkedIn for some time, almost five years. I started using it when I was looking for a new job and linked the fact that I could refer someone to my online resume without having to create a web site. Since I have moved around a bit and worked with many people dispersed throughout the US and Europe, LinkedIn was a great way to re-establish communication with former co-workers. LinkedIn has added RSS integration, displaying presentations, groups and calendar functionality for greater media integration.
Delicious: This is another tool that I have been using for a few years. I started using it solely to keep track of PR hits. By using tags I could get a quick summary of how many times a release or topic had been published. Previously I had created and filed a hard and soft copy, now I could bookmark, tag and save! Lately Delicious has added email capability to send articles and increased its community and contact tools. Plus I love the simplicity and clean design of their interface, no annoying stuff, just the links!