I have been peeking at Zimbra lately, a hot networked personal information manager. But instead of being run from your computer, it is run remotely (think Gmail+Google Calendar+Skype, and then some). There are serious advantages to this, starting with platform independence and reduced load on your local system resources. It also has advantages for collaboration, since all users are essentially working from one calendar and task list. And using the internet allows Zimbra to incorporate things like VOIP calling and shared documents.
As an enterprise solution, or even for a large law firm, Zimbra seems like a pretty great solution. But what about the solo or small-firm practitioner? Some of the above advantages are still there. Platform independence is a big one, for example. Zimbra also would allow a solo practitioner to more easily employ temporary or remote employees by making communication easy. Why run five programs to stay in touch when Firefox will do it all?
It also helps further the mobility dream. With Zimbra, you can even keep all your files on the server. Just log in from wherever you are (Portable Firefox should ensure sufficient security) and go to work.