I use pivotal tracker.
It's not really a calendar based tool, but more a project resource management type thing.
These tracker type programs are often used for programming. For bug checking, beta testing and deployment etc, but I just steal the terms and give them my own meaning.
For instance in pivotal tracker (which also runs on iphone etc. . super handy)
I set up a project, then the individual tasks, then checklists that each task involves. The task is rated by difficultly - i.e how long I guess it will take me to accomplish.
If it is something that the producer of the project has asked for its a FEATURE, if its something that they havent but it is needed (i.e location recording, research) its a CHORE.
If theres some audio clean up needing doing its a BUG.
A cheap way of using a tool not meant for sound, but its proven quite interesting.
Theres also the brilliant completion section, meaning you can process what you have delivered, if it was accepted, and if not, why not?
Between this, and a copious amount of colourful mind mapping tools out their, I can try manage as much as possible.
although we all know it comes down to a lot of caffeine and 2 dozen post it notes in the end. :D