One of the features that I used to use in iCal that is missing from Google Calendar is task management. I used to create tasks for repeating items like, flea treating the dog or replacing the furnace filter. Also I would add entries for projects that I wanted to remember to do.
When I switched to Google Calendar, I created events for these items, as Google Calendar doesn't show tasks from shared calendars.
I heard about Remember the Milk on the dl.tv podcast today, and I decided to try it out. This site lets you manage your task list. In addition to using the web interface on the Remember the Milk web site, you can also add the tasks to Google Calendar and iGoogle.
The other cool thing is that it uses Google Gears, so you can access your To Do list even when you are offline.