For quite a while now, I have been using Plaxo to manage my contacts. The great advantage of Plaxo is that it syncs with a variety of software—I use it with Outlook (two different copies), Thunderbird (ditto), and Gmail. The mobile version of Plaxo is also fast enough that you can access it even from an older smartphone instead of your local contacts.
With Plaxo, you can also sync your Outlook calendar(s), tasks, and notes.
LinkedIn does much the same thing, along with the social networking aspect. Both LinkedIn and Plaxo will automatically update contact information for any of your contacts who are also members, which is a handy feature.
Enter a new player, Keepm, which focuses solely on contact management. It will import from Gmail, Yahoo!, LinkedIn, etc. For someone who just needs a central contacts repository and does not want to muck about with all the social networking that comes with Plaxo and LinkedIn, Keepm looks like a simple, lightweight, and central contact manager.