I personally like the following three LinkedIn applications:

1) WordPress Application:  with this application, you can pull your WordPress blog into your LinkedIn profile.  You add value and can showcase your expertise.

2) Reading List by Amazon: you let people see what you are interested in with this application.  Who knows, maybe the person looking at your profile read the same book and it is the connection point they were looking for.

3) Polls: you create your own poll and conduct with that your own research within your network.  It is a great tool to better understand your audience, especially if your network is targeted.