Plaxo has been an incredibly helpful tool for quite some time and it simply keeps getting better. A few weeks ago, I slammed my cell phone in the car door. It was a perfect hit, dissecting the phone into 2 physical pieces. I received a new phone (always get insurance!) the next day but I had lost my address book.
I did a quick search of Verizon's Applications and found that they had Plaxo as one of them. I loaded it up and now I'm able to synchronize my phone with Plaxo on the contacts of my choosing. It saved me months of answering calls not knowing who was on the other end.
Now comes Plaxo Desktop Notifier
When folks who I am connected with twitter, post to their blog, or make a number of other changes, I receive a desktop notification. Since I auto-follow on Twitter, Twhirl keeps pretty busy and I actually miss some tweets from folks in my network.
Although I have a ton of connections on LinkedIn, I find that Plaxo is much more of an asset to me since it keeps all my address books (and phone) synched up. I don't mind paying for the service, either. The piece of mind knowing that any of my computers or my phone could be lost and I still have my address book is piece of mind worth paying for!