Google Calendars

30 Boxes would be my choice, if I used online calendars. It has a very rich Ajax interface and is very easy to use. 

You can also try Planzo, Kiko and Spongecell. Most if not all offer an intuitive user interface, and can be custom fitted to your taste. 

I would also recommend Rainlendar, as an offline calendar. I've used this until recently and was very pleased. 

i've tried them both, and here is what i found out; 

30 boxes:

pros - great options like rss feeds, tagging and free language input.

con -  can't edit a single event which is part of a repeating thing.

google :

pro - don't have the con of 30 boxes.

cons - don't have most pros of 30 boxes. 

if it is importent to you, you might want to check options for importing/exporting and syncing with other calendars.