I'm not a huge fan of social networking, but I think a lightweight application that can do all this (is it possible to do this and still be lightweight?) is the future of email. I'd love to see something that could download and give me my phone's voicemail too.
I'm really against all this "everything should run in your browser" stuff that's going on right now. I'd love to see these features implemented as some part of a future email client, but a client like this needs to run persistently on a machine, and as such it really needs to be fast and slim, something that no browser out there provides.