At the Colligo website you can register for beta testing Colligo for SharePoint 2007, a synchronizing client tool for offline working with SharePoint. Although the offline capabilities in Office 2007 are improved a lot, Colligo claims the two-way synchronization with SharePoint sites.
We've been testing offline/client tools for SharePoint over the last few years and those tools are great in some cases. In my opinion however, first see what Office 2007 has to offer before installing 3rd party products. Outlook 2007 for example, has a great offline synchronization mechanism for documents. Read about it here, here and here. Also Groove 2007 can offer a lot when it comes to offline synchronization.