This online calendar application has almost all of the features that I need. Things like: group calendars, integration with Outlook, SMS notifications, good permissions, and search. It also has features that I like that weren't necessarily part of my requirements, like blog content, shared contacts, lists, and links, and tight integration with most cell phone providers to provide an application that runs on most cell phones.
I looked at Office Calendar by Lookout Software, and I liked the concept very much, but the price tag seemed a bit steep. All in all, I am quite pleased with the features of Airset, and the price of free is hard to beat.
Disclaimer The opinions expressed herein are my own personal opinions and do not represent my employer's view in any way.