@gapingvoid shared a link to mixin which does for time what dopplr does for geography. Check out the video.
You can add your activities by time using IM, twitter, email, SMS and say when you'll be available and where you'll be.
The video then takes you through an activity, what are you doing, where will you be and when will you be there and you can invite any of your friends. Each activity is a conversation incorporating images video and audio - very much like a life-stream feed. You can also discover other people - as you might expect.
I like it as a tool but I am too loyal to twitter to move to anything else right now.
mixin presentation from mixin on Vimeo.