My Gmail address book must be one of my most trusty contact lists that I have. I constantly refer to it because it's easy to use and intuitive.
However, it's also a pain to have to log into Gmail everytime I want to access a contact. iContact works by using Gmail/Google Apps contacts list as the central database for your contacts. You can still see and edit your contacts in Gmail but iContact allows you to have instant access to your contacts whenever you need them without logging into your Google/Gmail account - great for if you are travelling or don't have access to an internet connection. The program can also integrate with Microsoft Outlook to import your contacts directly from there and add them into Gmail. It also features integration with Skype and Google Maps and allows you to send contacts to your Satnav or TomTom device which is useful for contacting people on the move. iContact also doesn't care which machine or PC you are working on - you can access your contacts on all computers without the need to synchronize them.
This is not exactly a Web 2.0 app but it's about as close as you can come without actually being one and a much more convenient way to organise your contacts.