Well  as predicted, Calendars would be a big thing, in a previous post Google are just about to realease a calendar application called CL2 according to TechCrunch. The interface is similar to GMail so it'll get huge uptake as people are used to it. I guess the main question I have is can you get to it using a Web Service or API because you could use this calendar system to add, edit, insert and then pull out the information using RSS.
This basically is an extension of the work Apple did with iCal and I can see the potential for this to go nuts just because it's Google. shared calendars may be an interesting extra or even sharing different sections of your calendar by tags etc.