I have been using Google Calendar to map out my day for so long that I cannot really remember not using it. Even when I was a corporate drone, Google Calendar was my best friend.
Now that my time is my own (well, not really, but you know what I mean), the Calendar is even more vital to my productivity. I usually map out my tasks and the time allotted for each task the night before, so that once I am ready to work in the morning, I know what I have to get done within the day.
Google Calendar is particularly helpful because I tend to linger on tasks if left untended. Additionally, when new clients and projects come in, I need to be reminded of tasks and deadlines. I use the Google Tasks in conjunction with the Calendar, for more efficiency. I sometimes work with a guest posting service, so there are times when my load is off the charts, and I cannot count the times that Google Calendar and Tasks have saved my sanity!
As for Google Docs, I mainly use the spreadsheet to monitor the number of articles I have written for whom, the date published/submitted, links, amount due, and so on. I learned to do this early in my freelance writing career, and I know I would be a mess without it.
The best thing: Google Apps is free!