Google have announced and demoed a work in progress called the Wave. Essentially it is reconceptualisation of email and instant messaging and collaborative tools into one browser based application which will be open source.
i think it has huge potential as it mimics conversation rather than writing a letter and it is hugely flexible and powerful.
Google’s demo at I/O was videoed and you can watch it here. It runs for over 1 hour 20 minutes so be patient. It even crashes…..Every one will grab it when it is available, it is such an obvious next step.