Some, such as Lycos' MailCity and Netscape's NetCenter, offer limited data import features, but you have to first take the information in Outlook and save it as a comma-separated file. That isn't something I wanted to do on a regular basis. TrueSync is used by Yahoo and Excite, while IntelliSync is used by PlanetAll and Visto. TrueSync has the best controls and the most flexibility of how contact information is manipulated, IntelliSync has fewer features but is easier and more reliable to use. There are other web-based services that offer synchronization but these four are the leading contenders and seem to have the most active development.
Synchronization sounds simple but is actually very sophisticated.
I setup accounts on each of the four services, and did several imports and synchronizations. I also kept the accounts active for several months and was annoyed to find that each service draws spammers like flies to honey. Here are my results. Of the four services, Visto offers the most promise and is probably the one that I will use on a regular basis for my home contacts. I have lots of flexibility in how I setup my synchronization sessions, and can even schedule them on a daily or other periodic basis.