Not everyone thinks the days of e-mail are numbered. Robert Hamilton and Ben Bradley write in Darwin Magazine about the how ingrained e-mail is to how we work. And, rather than fight it, they suggest one direction may be in creating environments which work seamlessly with email. They focus on collaboration and mention Kubi specifically. There are others, such as Groove and a number of application-specific plug-ins.
The following paragraph is an entertaining description of the problem. Collaboration & The Problem With E-mail
Because of human communication habits (defaulting back to the most convenient method), the e-mail client for many has become a personal knowledge management tool â€$(D s(Bomething it was never designed to do. E-mail really is proof that you can hammer in a nail with a screwdriver. Despite the arrival of some truly amazing tech tools in recent years, e-mail remains the most pervasively deployed and used application. E-mail can be thought of as primordial human activity that links us to the internetworked ecosystem. As such, e-mail habits are the most stable habits among technology users.
(emphasis mine)