Looking for client APIs to access calendar information

["Neelima Kalidindi" <nkalidindi@xxxxxxxxxxxxx>]

Hi All,
      I'm doing a research on the availability of client APIs that can
      access calendar information (using the iCal/vCal standard
notations).
      Specifically, I'm looking for APIs that can query if a certain
user or
      group of users is free at a given time or time range. I'd really
      appreciate it if someone could give me any help in this regard
even if
      it
      means direction to some source where I might find such
information.

Thanks
Neelima.