Guy Kawasaki has written an interesting post about using LinkedIn. He writes that LinkedIn is a tool that is under-utilized (I am guilty of that), and he has compiled a list of ways to increase the value of LinkedIn:
1. Increase your visibility.
2. Improve your connectability.
3. Improve your Google PageRank.
4. Enhance your search engine results.
5. Perform blind, “reverse,” and company reference checks.
6. Increase the relevancy of your job search.
7. Make your interview go smoother.
8. Gauge the health of a company.
9. Gauge the health of an industry.
10. Track startups.
11. Ask for advice.

Some of this is relevant for job searchers only but there are good advice for the rest of us too. Guy elaborates more about each of these points at Ten Ways to Use LinkedIn.