Google Calendar
Seems like an obvious project for them doesn't it?
I use the Yahoo Calendar now, and it's pretty good, but one thing that annoys me is that it only supports Yahoo Messenger for IM alerts. I would expect that if Google did their own calendaring app, they'd allow alerts to go out at least through some open protocol like Jabber.
The only other service Yahoo offers that I find useful, that Google doesn't have, is the Yahoo Notepad.