Powerful To-Do List: Set reminders and appointments inside the spreadsheet. Attach files, collaborate with your team. It's much more than a spreadsheet yet very easy to learn.
Integrates with Google calendar for setting reminders and appointments, and with Gmail for sending emails from within the spreadsheet. 
Can be used also for project management, as a lightweight CRM (simple customer management system), and for anything else where one needs to collaborate with a team and track one's goals and what needs to be done.
Best of all - it's free!