Google Calendar has worked for me. I was not a Gmail user, but Google calender has forced me to shift to Gmail. The following are a few of my wishes of features in Google Calendar. (But i know Google people dont listen to me. Like in 'Mark All Read' feature of Google Reader. )
* Configurable duration of the day (eg. show only 7am to 10pm). This will fit the full calendar in my screen (no scrolling needed.)
* A public/shared view of the calendar. Will help me publish my busy/non busy hours.
* Previous 2 days and next 4 days view [ numbers should be configurable ]
* Integrate with Gmail/GTalk/GToolbar more closely
– from calendar give links to associated mail
– from gmail show calendar agenda in sidebar
* To-Do lists
* Set to-date equal to from-date by default if not available (eg. while adding event from mail)
* New entry / edit event details should also be popup ( like the event info popup )
* Multiple days (but not full day) event should be shown like single day event streteched across multiple days. (The current way of showing it in full day events is stupid.)
* Upload a file associated with an event
* Cell phone notification service in india and should be synced with a cell phones calendar 🙂 (like Active Sync in Outlook)
* Should avoid being classified as "web based email" in websense!