Such as calendars, todo apps, freeform "scratch" files, etcetera.
Personally I tried Google Calendar for a while and didn't like it. I currently use a plain scratch text file to hold a schedule/todos/etc, but the simplicity actually gets in the way with the schedule, enough that I'm looking for a better solution.
I use Gmail a lot so what works for me is the app Remember The Milk. On the desktop it loads a task list on the right side of the Gmail screen. I put daily tasks there but for meetings I still take the time to enter it in the google calendar