Google Calendar
Probably the best online calendar and Task-management tool. Simple, quick and has everything an average person may wish for: speed, simple design, and lots of useful features.
Main Features:
- View calendar by day/week/month/…
- Create multiple calendars and share them with others (coworkers, friends, family, etc)
- Let others not only view but also add entries to your calendar. Very convenient for project teams
- Upcoming Event reminders: get notified by email OR mobile text messages
- Subscribe to calendars of others and view next to your own
- Well-implemented Gmail integration
- Search Calendars
- Natural language event entry: Typing ‘dinner on March 13 with Susan’ will create ‘dinner with Susan’ on March 13
- and more …
- Demo Tour
Visit Google Calendar

I love google cal. I also love that they opened up their API so that companies can make wicked apps that improve and already awesome calendar.
I use Calgoo for offline access
I use GooSync for sync with my mobile
I use Remember The Milk for tasks
And there always seems to be another app coming out that makes use of this. Some suck, some show promise, and some are awesome.
-MT