gcalcli is a Python application that allows you to access your Google
Calendar from a command line. It's easy to get your agenda, search
for events, and quickly add new events. Additionally gcalcli can be
used as a reminder service to execute any application you want.

Features:

list your calendars
show an agenda using a specified start/end time and date
graphical calendar display (my paying homage to the remind application)
search for past and/or future calendar events
"quick add" new calendar events to your default calendar
run as a cron job and execute a command for reminders
work against specific calendars (by calendar type or calendar name regex)
config file support for specifying program defaults
colored output and unicode character support
easy integration within shell scripts, cron, screen, conky, etc

Not (Yet) Supported:

import meeting.ics Outlook events
add (non-quick) events with ability to set reminders, repeat, guests, etc
configurable reminders (i.e. 30 mins before event every 5 mins)
offline mode working from cached data

The HOWTO here explains options in detail. Taken from:
http://code.google.com/p/gcalcli/wiki/HowTo